To all that replied, I installed a brake controller - That requires the 2 Red wires under the hood to be connected & the 4 under the dash. There are no others that I know of.
I've included a pic of the C/B box under the hood and I do not see anything related to trailer, hitch, ... power. The few empty positions are 68 (Stud 1 - possible if referrring to socket on hitch), 65 (elec Run Board - don't have), 58 (ALC/COMP - NO idea what this is), and PWR/TRN is installed, whatever that is again.
So no open items to be connected outside of possibly position 68, but there is no documentation I have found yet. Even the GMC dealer has no idea - he's checking.
I'll get a 40A fuse and put it in pos 68 and check if it will work.
Update - That was it! So for anyone with a 2008-? GMC Sierra , probably Chevy Silverado too, Position 68 Needs a 40 Amp fuse to power 12Vdc to Trailer. Considering, the TV had a towing package option installed, having no trailer brakes or wiring connected is ______.
Beacause the TV has a standard HO Alternator and was idle for a week or so, the voltage was apx 14.95-15.05.