I have a 2006 GMC SLE 2500 HD with the Duramax and auto- 6 speed Allison. I bought the truck used and the previous owner did not have a brake controller in it as he pulled a boat with surge brakes. I took the brake controller and harness out of my 04 Chevy Avalanche. This was plugged in the Z85 position under the dash of my Avalanche and can be plugged in the same location on the GMC. I verified that the same harness will work on the GMC, but should I also be putting a fuse in the fuse box in the engine, and if so where?

40 amp fuse for stud #1 IIRC, might check also the harness did change, but I cant recall the year... Pin location changed..

Stud #1 40 amp supplies 12V to charge the trailer batteries etc.
Stud #2 30 amp supplies 12V to the controller for the trailer brakes.

Both fuse positions may be empty or have dummy fuses installed.

http://www.pirate4x4.com/tech/billavista/Chevy/tech/Prodigy/index.html