Find out where the wiring is hooked to for the trailer light wiring on the truck, I've seen some pretty wierd connections, everything from black tape to quick connections to no insulation at all. also wires which are not in wire loom and proprely secured can rub on body parts and the frame and wear through the insulation. the best connection is soldered with heat shrink tubing, the next best is a crimp connector with heat shrink tubing. The harness behind the radio and the aftermarket plug can still get pinched and short out, there is a dash light wire in the after market plug and it is often not connected as few stereo's have externally controlled light switches so alot of times it is forgoten, it should be capped if it is not being used. You can also unplug the plug and see if it still blows the fuse and that will tell you if it is in the stereo or plug. good luck, never rule any thing out until you have checked it I have wasted alot of time by assuming something was good without checking it!
Proto