I got a 93' F350 quad cab dually. It will intermittently blow the #4 fuse under the hood which knocks out the clearance lights on the blister fenders. I wouldn't care too much about it, but my trailer connector is wired to this circuit so I have no trailer lights and no red lights on my fenders. This causes the truck to fail safety inspection. I can't find any wiring diagrams for this truck, or figure out what else besides the lights is associated with this fuse. I'm at the point where I want to just shove a jumper wire in the fuse position so it flames the offending wiring and I can stop wasting my time.

Is the trailer hooked up when it goes out. I had a simular problem and it was in the trailer not the truck. I would replace the fuse and check everything and it would all work. then drive a few miles down the road and I would have no running light on the truck and tailer. I traced it to the trailer were I found a small rub mark through on one of the wires and it would only touch when I hit a bump or the maybe the wind would make it hit the metal of the trailer. I then black taped it till I had time to rewire the entire trailer with better wire. After that I have not had any problems