I have a 1989 F250 7.5L 460 and have a small electrical problem I can't seem to track down. When I was pulling my trailer up a bumpy road I lost my rear running lights on the truck which in turn affected the trailer also. I have front running lights, dash lights, brake lights and turning signal lights but no rear running lights. I've checked all of my fuses under the dash and they are all ok so I'm thinking a wire got fried between the front and rear but I'm not sure so I thought I'd jump on here and ask for some guidance.
Power in the battery draining maybe the alt going bad. the wire is a real pain my 1992 f350 was had by a knife nut cut wires under big time no fuel gauges lots of wires cut. need to check the brown wire under the truck this is the color of the wire for the running lights.
Running lights on truck and trailer lights out, check the connection where your trailer connector is hooked into your truck wiring harness, it could just be a corroded connection. Depending on the trailer connection type, if it's OEM then you should be able to unplug it and plug your wiring harness back together to see if you get running lights. If it's an Aftermarket connector, those usually have those crappy clamp over wire type connectors that have a tendency to go bad.