I have a 1993 F-250 with factory wiring for the 7 prong trailer wire. My running lights quit working recently on the trailer. I took a battery and hot wired the D prong on the trailer and they work. I checked the D prong on my truck and it is dead. All fuses in the fuse box check OK. Is their another fuse somewhere in the line to the back of the truck? Any help would be greatly appreciated.