Ok supper Dave , let's get this stright , when you turn the key to start the vehicle , what happens ? The starter spins the engine or doesn't spin the engine ? This would be a no crank no start condition . if the engine does spin an doesn't start an run this is a crank no start condition . Now which is it ? The factory radio is hooked to the serial data communication network , did you or who ever installed the aftermarket radio use a interface module ? Are you familiar with the electronics on this vehicle ? BCM - body control module , low current inputs from the ignition switch which determine what mode to put the vehicle in ?
Case 4 Data Bus Issues
The body control system consists of the body control module (BCM), communications, and various input and outputs. Some inputs, outputs and messages require other modules to interact with the BCM. The BCM also has discrete input and output terminals to control the vehicle's body functions. The BCM is wired to the GMLAN high speed serial data buss and the GMLAN low speed serial data buss and acts as a gateway between them. If the BCM does not communicate the vehicle will not start due to the inability of the engine control module (ECM)/powertrain control module (PCM) and theft deterrent module (TDM) to communicate without the BCM providing the gateway function.
Power Mode Master
This vehicles body control module (BCM) functions as the power mode master (PMM). The ignition switch is a low current switch with multiple discrete ignition switch signals to the PMM for determination the power mode that will be sent over the serial data circuits to the other modules that need this information, and so the PMM will activate relays and other direct outputs of the PMM as needed. Refer to Power Mode Description and Operation for a complete description of power mode functions.
Sorry but I am a GM Tech . have been for over thirty years , Cadillac , Buick an Chevy .audio buds think it's something else. Your audio buds don't have a clue ? When change something you better well know what your messing with. .