What is best way to find a ground short in big truck?

What system does it have ? Base or up level ,does it have a amp ? Do you know what a RPO code is ? Without testing no one could say for sure what the issue is . First off would need to know what system we are dealing with .
RPO Options
The entertainment system on this vehicle is configured with either a base or uplevel audio system. Both the base and uplevel audio systems contain a radio, antenna, and speakers.
The following shows the Enteratinment RPOs that are available for this vehicle:
• (UM7) AM/FM Stereo
• (U1C) AM/FM Stereo CD
• (US8) AM/FM Stereo, CD, MP3, RDS, EQ
• (US9) AM/FM Stereo, 6-Disc CD, MP3, RDS, EQ
• (UXQ4) Base Four Speaker
• (U79) Uplevel Four Speaker
• (WBM) Premium Audio
• (U2K) Digital Radio

Circuit Operation
Radio Power: The main radio power is supplied by the 10A RADIO (BATT1) fuse in the body control module (BCM). Radio switch-on power, 12 volts in ACC, RUN or RAP, is supplied by the 10A RADIO (ACC.) in the BCM.
Radio Grounds: The main radio ground provides a ground for the radio circuits. An additional braided ground strap provides a shielding radio case ground to reduce EMI noise. Both grounds are connected together inside the radio.
Radio Speaker Outputs: At zero volume, the plus (+) and minus (-) speaker outputs are both approximately 5-6 volts, measured to vehicle ground. If a plus or minus for any speaker output is shorted to ground or voltage, the radio circuitry will turn OFF all 4 speaker outputs for component protection. Above zero volume, the plus and minus change to create a voltage difference between each other, to drive the voice coil of the speaker.
Radio Park Lamp Input: The radio park lamp input allows the radio to sense when the vehicle parking lamps are ON. When the parking lamps are ON, this circuit goes to battery voltage, and the radio display back-lighting switches from full bright to the brightness level determined by the I/P illumination input.
Radio I/P Illumination Input: A pulse width modulated (PWM) voltage for instrument panel (I/P) illumination is provided to the radio I/P illumination input. The Radio uses this input to directly illuminate the radio buttons and adjust the radio display back-lighting when the park lamp input is ON.
Amplifier Power: The main amplifier power is provided by the 20A PREM AUDIO fuse in the instrument panel fuse block..
Amplifier Radio Speaker Inputs (WBM only): The radio speaker outputs, at a reduced output level for amplified systems, are the amplifier inputs. The amplifier boosts these inputs and outputs them to the vehicle speakers. Speaker plus and minus circuits from the radio change to create a voltage difference between each other. If one speaker plus or minus is open between the radio and the amplifier, the input to the amplifier is approximately half because only one of the circuits is changing. The speaker for that channel then operates at approximately only half the normal volume.
Amplifier Speaker Outputs (WBM only): At zero volume, the plus (+) and minus (-) speaker outputs are both approximately 5-6 volts, measured to vehicle ground. If a plus or minus for any speaker output is shorted to ground or voltage, the amplifier circuitry will turn OFF either the front outputs or rear outputs for component protection. Above zero volume, the plus and minus change to create a voltage difference between each other, to drive the voice coil of the speaker.
Amplifier Radio-On (WBM only): When the radio is ON, this circuit is pulled to 12 volts by the radio. The amplifier switches ON when this circuit is 12 volts and switches OFF when this signal is 0 volts.
Remote Radio Audio Signal Inputs (UE1 only): Audio output from the OnStar® communications module connects to the remote audio signal inputs of the radio. When the cellular telephone mute signal goes to 0 volts, the radio over-rides any other audio signal and uses these inputs as the source for output to the speakers.
Cellular Telephone Mute (UE1 only): The OnStar® communications module uses the cellular telephone mute signal circuit to over-ride the radio for OnStar® communication. When cellular telephone mute is not active, this circuit is held at 2 volts by the radio. When the cellular telephone mute signal is pulled to ground, the radio over-rides any other audio signal and uses the remote audio signals as the source for output to the speakers. If the radio was OFF when this circuit is pulled low, the radio will turn ON. Additionally, the radio fades the speakers to full front, adjusts the volume to an initial audible level, and sets an AutoTone designed for optimal use with OnStar®. When the mute signal is no longer pulled to ground, the radio returns to the mode it was in previously.

Hooking up a professional or factory scan tool that can read radio codes would be one of the first test's to do .
DTC B1025, B1035, B1045, or B1055 (w/out RPO WBM)
DTC B1025, B1035, B1045, or B1055 (w/ RPO WBM)
DTC B1030, B1040, B1050, or B1060 (w/ out RPO WBM)
DTC B1030, B1040, B1050, or B1060 (w/ RPO WBM)
DTC B1259
The following conditions are present for 250 ms or longer:
B1025 - B1055 01
One of the speaker positive output circuits is shorted to battery.
B1025 - B1055 02
The speaker positive output circuit is shorted to ground.
B1025 - B1055 04
The speaker positive output circuit is open.

Your best bet , take it to a qualified repair shop .

Speakers get power from the amp if your system has one or from the radio , if the tweeters work it's not a fuse problem . You like listen to your stereo loud . Blew a channel in the radio or amp ?
Take it and have it checked for DTC'S - diagnostic trouble codes
DTC B1025 01: Audio Output 1 Left Front Circuit Short to Battery
DTC B1025 02: Audio Output 1 Left Front Circuit Short to Ground
DTC B1035 01: Audio Output 2 Right Front Circuit Short to Battery
DTC B1035 02: Audio Output 2 Right Front Circuit Short to Ground
DTC B1045 01: Audio Output 3 Left Rear Circuit Short to Battery
DTC B1045 02: Audio Output 3 Left Rear Circuit Short to Ground
DTC B1055 01: Audio Output 4 Right Rear Circuit Short to Battery
DTC B1055 02: Audio Output 4 Right Rear Circuit Short to Ground

Each of the audio output channel circuits (+) and (-), at the radio and audio amplifier (if equipped) have a DC bias voltage that is one half of the battery voltage. When using a DMM, each of the audio output channel circuits will measure approximately 6.5 V DC. The audio being played on the system is produced by a varying AC voltage that is centered around the DC bias voltage on the same circuit. The AC voltage is what causes the speaker cone to move and produce sound. This voltage will vary depending on what type of audio is being listened to, talk or music, and type of music, as well as the volume setting of the system. Low volume and talk settings will measure around 1V AC and less, where constant music may measure 3 V AC and slightly higher.
Do you know how to do DC voltage testing ?
Without Amplifier without UK6
Ignition OFF, disconnect the harness connector at the appropriate speaker.
Radio ON, test for 5.0-7.5 V between the signal circuit terminal A and ground.
? If less than the specified range, test the signal circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the radio.
? If greater than the specified range, test the signal circuit for a short to voltage. If the circuit tests normal, replace the radio.
Test for 5.0-7.5 V between the signal circuit terminal B and ground.
? If less than the specified range, test the signal circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the radio.
? If greater than the specified range, test the signal circuit for a short to voltage. If the circuit tests normal, replace the radio.
If all circuits test normal, replace the speaker.

Apparently there is a "dead short" that is draining the battery. If a power wire or connection (plus side) is in contact with a ground wire or source this will create a short that drains the battery. When the battery is charged feel the wires and all connections to the radio and you may feel heat from the bad connection. Best bet is disconnect the entire set up and start over. It is best if the wires stay color coded (if you replace any). This dead short could cause a fire or damage any other wiring it is connected to. If the prior owner ran a new wire from the battery make sure those connections are tight and not rubbing any bare wires that could touch a ground source. (Most metal parts in vehicles are connected to the ground side of the battery. Also check your speaker connections from the unit to each speaker. Those wires can not touch each other or any metal. I hope this helps.

What is causing that irritating noise? ' DSMtuners

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The chimes an turn signal noise come out driver door speaker . Both front speakers not working could be a blown circuit fuse inside the radio or just blown front speaker channel . Your best bet , take it to a GM dealer or someother qualified repair shop . You have no idea of the complexity of this . The radio can set DTC'S - diagnostic trouble codes , believe it or not !
DTC B1025, B1035, B1045, or B1055
DTC B1025 01: Audio Output 1 Left Front Circuit Short to Battery
DTC B1025 02: Audio Output 1 Left Front Circuit Short to Ground
DTC B1035 01: Audio Output 2 Right Front Circuit Short to Battery
DTC B1035 02: Audio Output 2 Right Front Circuit Short to Ground
DTC B1045 01: Audio Output 3 Left Rear Circuit Short to Battery
DTC B1045 02: Audio Output 3 Left Rear Circuit Short to Ground
DTC B1055 01: Audio Output 4 Right Rear Circuit Short to Battery
DTC B1055 02: Audio Output 4 Right Rear Circuit Short to Ground

Each of the audio output channel circuits (+) and (-), at the radio and audio amplifier have a DC bias voltage that is one half of the battery voltage. When using a DMM, each of the audio output channel circuits will measure approximately 6.5V DC. The audio being played on the system is produced by a varying AC voltage that is centered around the DC bias voltage on the same circuit. The AC voltage is what causes the speaker cone to move and produce sound. This voltage will vary depending on what type of audio is being listened to, talk or music, and type of music, as well as the volume setting of the system. Low volume and talk settings will measure around 1V AC and less, where constant music may measure 3V AC and slightly higher.
DTC B1287 01: Amplifier Control Signal Circuit Short to Battery
DTC B1287 02: Amplifier Control Signal Circuit Short to Ground
DTC B1287 04: Amplifier Control Signal Circuit Open

The radio provides a remote amplifier mute output circuit to control overall muting of the remote amplifier. The amplifier receives pulse width modulated (PWM) signals on the control circuit at varying duty cycle percentages for controlling muted and unmuted functions of the amplifier. Circuitry internal to the radio monitors the remote amplifier mute circuit for faults and sets the appropriate DTC when faults are detected.

when the radio is OFF sound comes out of the rear speakers when the blinkers are used ! There isn't a turn signal flasher on your vehicle ,( like most all newer vehicles ) It's done electroniclly by a control module . In the case of your vehicle , the BCM - body control module . It doesn't matter if the radio is off . All the Audible warnings come out of the radio speakers . Headlamps left on , key in ignition etc... You have no idea of the complexity of your vehicle , it's a rolling computer , with a serial data network for the modules to talk to each other , share info.
When the turn signal switch is placed in either the left or right position, a ground signal is completed from the turn signal switch to the BCM. When the hazard flasher switch is activated, a ground signal is completed from the turn signal/hazard switch to the BCM. The BCM then sends an ON-OFF voltage signal through the fuses to the appropriate turn signal and side repeater (export only) lamps or to all of the turn signal fuses for hazard operation.
Really don't think you have electrical short . Computer glitch maybe , radio problem . How ever , these system's are not for the untrained to mess with . Your best bet , the GMC dealer !

You are not the only one with this problem... Alot of 01' impalas had problems with the radio. You could take it out to see if there are any loose wires or something grounding out but if it was me I would upgrade to a CD player that has bluetooth and Aux input and some of the newer stuff with it. Also don't foget that it is possible that at or between the speaker connection there could be a short or something grounding out to cause the problem. But no matter what, the dash has to come off so why not put in a new one while you are in there getting your hands dirty. Also Wal-Mart sells a dash kit for every year car/truck but please install it your self don't go and get ripped off by best buy or one of these hole in the wall stereo places. P.S. Take Off The Battery Terminals Before You Do Anything!!!!!!!

bad ground wire

sound like a speaker wire is grounding out somehow

your speaker is burnt out , see an auto electritian cos your car has just short cucuited and is very danderous the whole car could catch fire,