Starter turning in on postion
some of the later PCM's had a CRANK REQUEST wire, and the PCM controlled the starter relay based on signal it got on CRANK REQUEST circuit.
Without knowing year of the harness/pcm impossible to know what your issue might be.
attached is the starting circuit for 2005 vortec truck.
I think it is the ignition module. There is no resistance when the key is turned and it doesn't rebound back to run when it is turned to start. The key can be turned until it stops at the end and stays there, it doesn't rebound.
The part I can't figure out is when the PCM is hooked up the starter will turn, but in the ON position. When the PCM is disconnected the starter does nothing, but the radio and accessories will work.







