Did your truck come with a towing package, and have you towed with it before?
Mine supposedly came with a full towing package including brake controller - but did not work, I forget if it was brakes, running lights or whatever but I distinctly recall something was inoperative - the truck was brand new.
Took it back to the dealer and it turned out there were extra fuses that had to be installed or wires connected, they didn't give me exact details - in any event that was the problem!
You can get 7 pin socket testers to stick in the truck socket - they have LED lights to confirm power on the various pins. Likely Harbor Freight would have them in the U.S.
Brian & Connie Mitchell
2005 Classic 30'
Hensley Arrow / Centramatics
2008 GMC Sierra SLT 2500HD,4x4,Crew Cab, Diesel, Leer cap.