My '98 f150 came with a factory wiring harness(4 plug) simple wring or so I thought. Several weeks ago I lost my brake/blinker lights on my boat trailer, then my running lights. I went to the FORD dealer, was told a fuse and the flasher relay would correct the problem. Now I have running lights but still no blinker/brake lights on the trailer. I took a light tester up to the connector found the correct color codes and have blinker/running lights with the light tester, but the only light out of the factory plug is the running lights. I bypassed the factory wiring took a wire from the connector straight to the trailer wire and still dont have blinker lights. Any ideas? The ground wire is good, with out it hooked up no lights at all.

 

I had a trailer wiring harness short out (moisture got in the open end when not in use?) and it messed up the blinkers on my Ranger. Had to replace the wiring harness in the steering column to fix it. That was a pain to track down.

Sounds like your harness may have shorted something further back up the line on your truck.

 

I am at a total loss, the FORD dealer advised to check all the fuses with a light tester and that checked out, so tomorrow I'll go to FORD and spend $37.00 for a diagnosis. This will at least tell me where the "short" is and I can go from there.

Thanks for your reply

 

If all the lights work on the truck I would check the trailer wiring fuses under the hood. There are 3 or 4 that control trailer lights only. The fuses under the dash are for lights on the vehicle.

 

On the fullsize trucks, the trailer turn/brake bulbs are protected by engine compartment fuses G & F.