How do I wire my MTX 7801 CLASS D MONO BLOCK AMP for best sound

You wired color ? Hooking up an aftermarket radio on your vehicle requires a special interface module , that may not be the name it's called . You better check at a stereo shop that installs radios . Your vehicles radio is connected on what is referred to as class 2 serial data communication network .
Class 2 Serial Data Link
The following modules use the class 2 serial data link to communicate with each other:
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The body control module (BCM)

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The digital radio receiver (DRR)

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The electronic brake control module (EBCM)

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The inflatable restraint sensing and diagnostic module (SDM)

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The powertrain control module (PCM)

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The radio

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The vehicle theft deterrent (VTD)

The class 2 serial data link also allows a scan tool to communicate with the above modules for diagnostic and testing purposes.
The class 2 serial data link on this vehicle is configured in a star pattern. There is no redundant path for the serial data to flow through. If a class 2 serial data circuit develops an open condition, the module connected by that circuit will not be able to communicate.
If not hooked up right the vehicle may not even start .

Backup Lamp Circuit Description
Battery positive voltage (B+) is supplied at all times to the REVERSE relay coil through the rear fuse block. Battery positive voltage (B+) is supplied at all times from the rear fuse block NSBU Fuse to the REVERSE relay switch. The powertrain control module (PCM) sends a class 2 message to the rear integration module (RIM) indicating that the gear selector lever is in the REVERSE position. The dash integration module (DIM) sends a class 2 message through power moding that the ignition switch is in the ON position. When these conditions occur, the RIM grounds the REVERSE relay coil. The coil is energized, closing the switch contacts. This supplies battery positive voltage (B+) to the backup lamps. The backup lamps are both permanently grounded, so both backup lamps have power and ground, and then light.

Do you know about class 2 serial data communications between control modules ? Do you know there is a fuse box under the rear seat ?

This is information taken from a Buick service repair manual !

Park Lamps
The dash integration module (DIM) supplies battery positive voltage (B+) to the PRK LP relay coil. When the headlamps or park lamps need to be on or when the ignition switch is in the ON position. Battery positive voltage (B+) is supplied at all times to the PRK LP relay switch. When the park lamps are to be active, the DIM grounds the PRK LP relay coil. The coil is energized, closing the switch contacts. Battery positive voltage (B+) is supplied to the rear fuse block RH PARK LP Fuse, LH PARK LP Fuse, and DIMR Fuse. The LH PARK LP Fuse supplies battery positive voltage (B+) to the LH park lamps. The LH PARK LP Fuse supplies battery positive voltage (B+) to the LH park lamps. The LH and RH park lamps are permanently grounded, so all park lamps have power and ground, and then light. The DIMR Fuse supplies battery positive voltage (B+) to the DIM park lamp input for interior lamps dimming, the license lamps, the underhood lamp, and the cornering lamps. The license lamps are permanently grounded, so both license lamps have power and ground, and then light.

The body control system consists of 3 modules:
• The dash integration module (DIM)
• The instrument panel module (IPM)
• The rear integration module (RIM)
Dash Integration Module (DIM)
Battery voltage is provided from the rear fuse block to the DIM. The module ground is wired to ground. Battery positive sense voltage is provided from the rear fuse block to the DIM. This battery input is referenced to the ground sense input at DIM which is wired to the battery negative terminal and to ground. These sense inputs are physically connected to the battery positive and negative terminals for accurate battery voltage readings. The DIM is wired to the class 2 serial data line. The various DIM input and output circuits are described in the corresponding functional areas as indicated on the DIM electrical schematics.
Instrument Panel Module (IPM)
Battery voltage is provided from the underhood fuse block to the IPM. Ignition voltage is provided from the rear fuse block to the IPM. The voltage inputs are referenced to the module ground. The IPM is wired to the class 2 serial data line. The various IPM input and output circuits are described in the corresponding functional areas as indicated on the IPM electrical schematics.
Rear Integration Module (RIM)
Battery voltage is provided from the rear fuse block to the RIM. Ignition voltage is provided from the rear fuse block to the RIM. These voltage inputs are referenced to module ground. The RIM is wired to the class 2 serial data line. The various RIM input and output circuits are described in the corresponding functional areas as indicated on the RIM electrical schematics.
Rear Integration Module (RIM)
The RIM functions include the following functions:
• Heated seat controls
• Rear defogger control
• Park relay controls
• Various controls for the interior lamps
• Reverse control for the backup lamps
• Rear compartment lid release controls
• Retained Accessory Power (RAP) control
• Ajar switch and tamper switch inputs from the rear compartment lid
• Vehicle leveling
Your best bet would be to take your vehicle to a qualified repair shop ! No offence but this is complex an a factory or professional grade scan tool is needed , could have DTC'S - diagnostic trouble codes stored .
Body Control System Description
The body control system consists of the following 3 modules:
• The dash integration module (DIM), located behind the instrument panel, under the right dash sound insulator.
• The instrument panel module (IPM), located behind the instrument panel, under the left dash sound insulator.
• The rear integration module (RIM), located behind the rear seat back cushion (upper, left).

If your wires are hooked up correctly then it is possible you have blown a channel in your amp or head unit , the amp does have its own fuse but most do not have a fuse for each channel it either works or doesn't . Only other possibility is somehow the amp has been switched to mono .or "bridged" these settings are used when the amp powers only one channel or a subwoofer

your radio is either MONO, depending on age, or it is STEREO , poor sound is more than likely due to the speaker/s condition , the radio broadcasts are usually in stereo now so maybe you've not got the radio set up for the best sound etc , what is the make of the radio and age

grey/yellow

Sounds like there is a bad connection, (positive or negative) From battery, Check and clean all connections. If wires and connections are good check for loose wires or fuses at fuse block.

no the factory alternator will not have nearly enought power to operate this amp. Yes the alternator is BOSCH and it has a rated output of 75 Amps, u need at least 140 amps, any really good auto electric shop can supply a rebuilt high output alternator custom built, but it will be very expensive.

sound like the head unit's out put mosfet is going bad, but i have also ran in to this problem when it was a speaker wires shorting out. so first check the connection on your rear speakers.

To isolate what speaker is what set of wires, I always use a AA 1.5V battery and touch one end of each set of wires on each end of the battery, and you'll hear a crackle from the speaker they belong to, then I label it. Can use AA,AAA,A,C,D battery doesn't matter as long as 1.5v. Don't know if that's what you were looking for or not. Old trick from years ago.