I have a 98 F150 and tow a boat. The problem I have encountered is the running lights on the trailer will work but the brake lights and turn signals will not operate, they do however work on the truck. I have hooked up the boat to another vehicle and all lights operate correctly so I know the problem is not in the trailer wiring. I bought and installed a new factory harness and recepticle but the problem still persists. All of the fuses are good as well. Anyone have a suggestion?

 

There may be an extra relay and fuse you need to pass the power back to the connector.
The other truck you tried the trailer on probably has all the electrical hardware in place.

 

Any clue how to determine which relay may carry the current to the connector? This is probably the problem. They all worked at one time and then one day just quit?? Again, all fuses are still good inside the cab and under the hood.

 

Had a simular problem after my old expy had been in for service. Is it possible the connector on the BACK of the trailer connector has come loose???