No power inside the cab pw, pl, rd, cluster panel, alternator keeps shorting out, code p0621

Finding The Source Of Electrical Shorts In Your Car
This is the easy way to find shorts !

Disconnect alternator and replace blown power fuse and see if it is the actual alternator short or harness related. Always check your ground wire for corroision. Ground wires cause resistance problems and cause premature failures of electrical components.

Could have a wiring problem , a wiring pin backed out of a connector ! If you don't know about automotive electrical an wiring diagrams an electrical wiring trouble shooting I suggest you take it to a ASE certified repair shop .
Instrument Cluster and Panel Illumination The instrument cluster and panel illumination is used to provide backlighting to the instrument panel, switches, and control components when the headlamp switch is in either the PARKING LAMPS ON or HEADLAMPS ON position. The level of intensity is adjustable by pushing the instrument panel dimmer switch either up to increase intensity or down to decrease intensity.
The dimmable backlighting components are:

• Instrument cluster
• Hazard switch
• Rear defroster switch
• Message center switches
• Parking aid switch (if equipped)
• Transmission range selector
• Fog lamp switch

The non-dimmable backlighting components are:

• Driver front door lock and power window control switch
• Passenger front door lock and power window control switch
• Driver rear door power window control switch
• Passenger rear door power window control switch

Principles of Operation
The instrument cluster sends a signal to the instrument panel dimmer switch. The instrument cluster provides a variable pulse width modulated (PWM) signal to the dimmable backlighting components in accordance with the position of the instrument panel dimmer switch.

1. Verify the customer concern.

1. Visually inspect the following for obvious signs of mechanical or electrical damage.
   
   Visual Inspection Chart Mechanical Electrical
   • Instrument panel dimmer switch
   • Smart junction box (SJB) fuse(s):
     • 7 (10A)
     • 15 (30A)
   • Circuitry
   • Illumination bulbs
   • Instrument cluster
   • SJB
   1. If an obvious cause for an observed or reported concern is found, correct the cause (if possible) before proceeding to the next step.
   1. NOTE: Make sure to use the latest scan tool release software.
      If the cause is not visually evident, connect the scan tool to the data link connector (DLC).
   1. NOTE: The vehicle communication module (VCM) LED prove-out confirms power and ground from the DLC are provided to the VCM.
      If the scan tool does not communicate with the VCM:
      • Check the VCM connection to the vehicle.
      • Check the scan tool connection to the VCM.
      • Refer to Section 418-00 , No Power To The Scan Tool, to diagnose no communication with the scan tool.
   1. If the scan tool does not communicate with the vehicle:
      • Verify the ignition key is in the ON position.
      • Verify the scan tool operation with a known good vehicle.
      • Refer to Section 418-00 to diagnose no response from the powertrain control module (PCM).
   1. Carry out the network test.
      • If the scan tool responds with no communication for one or more modules, refer to Section 418-00 .
      • If the network test passes, retrieve and record the continuous memory DTCs.
   1. Clear the continuous DTCs and carry out the self-test diagnostics for the instrument cluster.
   1. If the DTCs retrieved are related to the concern, go to the Instrument Cluster Diagnostic Trouble Code (DTC) Index. For all other DTCs, refer to Section 419-10 .
   1. If no DTCs related to the concern are retrieved, GO to Symptom Chart .

Symptom Chart Condition Possible Sources Action

• One or more module-controlled illumination source(s) is inoperative

• Circuitry
• Inoperative illumination source component

• Go To Pinpoint Test A .

A2 CHECK VOLTAGE TO THE INOPERATIVE ILLUMINATION SOURCE

• Key in OFF position.
• Disconnect: Inoperative Illumination Source Connector.
• Key in ON position.
• Place the headlamp switch in the PARKING LAMPS ON position.
• Measure the voltage between the inoperative illumination source, harness side and ground according to the following table:
  
  Illumination Source Connector- Pin Circuit Rear window defrost switch C2340-3 19 (LB/RD) Hazard flasher switch C2039-8 19 (LB/RD) Parking aid disable switch C2339-3 19 (LB/RD) Message center INFO switch C269a-3 19 (LB/RD) Message center SET switch C269b-3 19 (LB/RD) Transmission range selector C2096-4 19 (LB/RD) Fog lamp switch C2341-3 19 (LB/RD)
  
• Is the voltage greater than 10 volts?

have your alternator rechecked it could be bad even tho its is new this is very common .

The codes are for two different circuits that involve the alternator and the computer. One is for the field circuit and the other is the lamp indicator or gauge circuit. The PCM controls the voltage regulator inside the alternator.
You would need to take it to a shop that has the books, or buy a used copy of the GM shop manual to get the test procedures for both codes.
The problem could be in the wiring harness, or the computer.

check if the fuse for the gauge panel / radio is properly seated/installed. check for loose wires as well. all of this can be seen inside your kick panel

alternator bad thats why no ligths to cluster it comes from hot wire from alternator

u can read the chart of the fuses on the inside of the fuse box

Interior fuse box fuse no.33(15amp) supplies power in run/acc postion to BK/PK wire pin 19 of cluster.
Interior fuse box fuse no. 45(10 amp) supplies power in the start or run postion on the RD/YE wire pin 20 of cluster.
Interior fuse panel fuse no. 41(10amp) supplies power to the battery saver relay on LG/OG wire pin 11.
Interior fuse panel fuse no. 35(10amp) supplies power to interior lamp relay on BK/LG wire pin 12.
The last 2 are hot at all times.

Remove power to all aftermarket components, if you know how to use a multi-meter connect to power source inside vehicle.. Drive normal, if voltage drops below 12v while driving, it is faulty alternator. If bat light comes on and voltage is above 12v the plug on the alternator might be correded and throwing faulty light, or bad ground in gauge cluster...