No PCM comms means the PCM doesn't have power or the CAN, Class II serial data, power, or ground are not present on the diag port. 2007 & earlier likely do not have CAN.
Check for power and ground on the diag port with a meter. DO NOT plug a meter into terminal 2, 6, or 14 of the J1962 port.
Check for power at the PCM connectors. You're going to need to get some service information on this. Probably will require a purchase from Alldata DIY or Mitchell or ...
You can check at terminal 2 for serial data with a logic probe or oscilloscope. DO NOT plug a meter into the serial line. This is not RS232 serial. You can let the magic smoke out of the UART on the PCM.
This is the government mandated DLC, OBD II port, SAE J1962 port, Diag port, ...
- Terminal 2 is the Class II Serial data from the PCM BCM Radio ...
- Terminal 4 is the diag port ground
- Terminal 5 is the PCM ground
- Terminal 16 is BATTERY positive power
If the truck has GMLAN (CAN) like my 2009 Impala
- Terminal 6 is CAN +
- Terminal 14 is CAN -