All lights worked fine on my old truck ( 05 GMC ), but my new 2013 F-150 with factory 4 & 7 pin plug, the running and side lights won't work. Trailer has 4 pin connector. Tried both plugs (with dongle) and I get the same results. Turn signals and brake lights work.

Only thing not factory is I had an ARE bed Cap installed with LED interior bed light. I noticed the installer took a bulb out of the cap high 3rd brake light, and took the cover off the left tail light to do something with the wires. I do not know what he did.

I'm not very knowlegable with electrical problems, but do you think it's related to the instalation of my cap? If so, I'll go back to cap installer, or to dealer under warrentee?

 

First - check the trailer on a different truck to make sure its not the trailer
(did the ground wire break on the trailer? They tend to do that...)

If the trailer is OK, then its probably in the truck.

You could check the pins with a multimeter (if you have one and the knowledge to use it)
Or go back to the cap installer and try to get them to fix it. They modified the wiring so its possible they separated one of the wires to your trailer hookup.

The dealer would be my 3rd choice. If the truck doesn't work because you broke it (by taking it to the cap installer) then it is highly likely they will charge you for the repair.

 

Be careful pulling fuses. My dad had an issue with his trailer lights, and in the process of checking fuses, he pulled one that activated the anti-theft system. Had to have his truck towed to the dealer after that.

 

Check the fuses under the hood.. I pulled a Uhaul trailer and i only had brake lights and no running lights or signals. Turned out a fuse popped.. Replaced it and good as new.