First that fuse is for the power to the tow cable plug-- leave that out for now-! It is only to power and charge if there is a trailer in tow.
Check the ground cable at the frame under the driver's seat. Clean it and make sure it is secured good.
Does it work OK with the light off-? If so- could be a short in the trailer plug-- or the cable that is behind the driver side-- on top of the gas tank-- (That is for a 5th wheel plug if the person wants to add one.)
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.--- Jerry