Hi could someone help. I have just installed a new wiring harness for trailer towing. My truck had the trailer towing package. Everything works except the left signal and left stop light. It's driving me mad. Dealer says it's got to be at the main plug along side the frame. But the lights work there and on the truck itself. but not at the trailer. I just had the electric brake controller replaced as well.

 

Are you using a test light... and are you talking about the 4-pin or 7-prong connector?

 

Have you checked the fuses in the power box under the hood? The trailer wiring fuses are in there.

 

TY all so much. I didn't know there was a fuse box under the hood. But yes that was the problem. Ford here had no idea what the problem could be. Thx again.