He has a 06, and those do not have the integrated brake control system.
The fuse is in the under hood fuse box. You should take the trailer plug apart and check the wires.. might have a shorted/loose wire there. Another place-- check on top of the fuel tank-- the 5th wheel wiring harness is there and can get bare wires. It is a long harness without a end, just taped off wires.
Chevy 04.5 LLY, 2X, shell, XDR, MikeL Heavy duty up-grade, 6spd, TransGo, deep pan, de-badged, 65 Gallon replacement Aero tank, LBZ-mp, Kennedy pump, shimmed, Pre-fuel filter, tow mirrors, BU cam, gauges, GPS, Sirius, bags, custom grille, fender vents, HD trans cooler- Curt front hitch, footwell lights, load lights, Studs, head gaskets.