I have an OEM 7/4 pin trailer connector on 2002 f-150. Recently I lost my running lights to my boat trailer. Both signal and brake lights work. I probed the pins on the 7 pin connector and all are hot except for the running light pin. I unhooked the harness under the truck and probed there. The running light pin was hot. So I assumed that the harness was bad and went out and purchased a new one. After the instaling the new harness, I am back to right where I was before. Both signal and brake light pins are hot but not the running light pin when the lights are on. I checked the fuse box under the hood and where there are supposed to be relays for the trailer tow package these slots are empty. All lights on the truck are working so I don't think any fuses are blown. Very confused and don't know what to try next.

 

Check the fuse box under the hood. That 's were the problem is most of the time

 

If the lead is hot where the harness plugs in but it isn't hot coming out of the harness, then obviously the problem is where the harness plugs in to the truck. Look for a loose connection or perhaps the plug isn't making good contact. But if the wire is hot at the back of the truck then it ain't a fuse problem.