Get the BCM and plugs from under dash, plus shift lever from same truck if ya can. There are two sets of high speed CANbus wires on the BCM, one set goes out to the glow plug harness, then onto ECM & TCM. That's set ya need to connect up.
Then power the BCM same as truck would be. The tap shift / tow haul switch wires run directly to the BCM. When you push a button for M mode, the BCM sends a CANbus data command to the TCM to change shift tables.
The BCM is no more than a solid state relay bank and analog to digital converter. When you flip a switch or button, it tells a relay to turn on, or send that signal to another module. In your case the TCM. So just need to plug it all back into the system and it works.. But everything needs to match models and VIN's.
Ya still disable VATS in the ECM unless your pulling the keys and security module from same truck to match. But lets say you swapped the whole steering column, then you could really update things with volume control buttons and such. The BCM is the main brain for controlling everything non-powertrain in the vehicle.
I have all the pintout and harness diagrams from GM. Shoot me a PM if ya need them.
