I have a 1999 K2500 (OBS) that I recently swaped an 8.1L i to. The engine came out of a 2001 Silverado (NBS). I retained the 4L80E/NP241C that was already in my truck. I was fortunate enough (I think) to located a wiring harness from a 2001 C3500 (OBS) which of course was 2wd. Everything plugs in just like it was made for this truck and engine combo except the transmission/transfer case. The last thing I need to figure out is how to wire the 4wd transmission and transfer case. The C3500 harness has plugs for the Neutral Safety Switch on the side of the transmission but I have one remaining 4 wire plug that I have not yet figured out. The 4 wires on the truck side one goes to a sensor on top of the transfer case (I think range selector), two go to the 4wd actuator up front, and the 4th i have not been able to track down. Are there any transmission/transfer case experts in the group or anyone who can point me in the direction of a good wiring diagram? I have traced the wires through the old 454 harness and they terminate as follows; Pin A - goes to pin #23 in black ecm connection (old harness) Pins B&C - both black wires, appear to be ground. One ties into another ground wire about half way up the harness and the other goes to the plug on the firewall and connects to D6. Unfortunately I don't know which one is goes where. Pin D - Also goes to firewall plug pin E5. Any help would be greatly appreciated.