So, I have a 2005 f150 I've had for some time. It has a factory 7 and 4 pin that has always worked well. Last time I hooked it up though the running lights only come on when you hit the brake even if headlights are off and the right turn signal makes every light on the trailer blink in sync. Auxillary power and electric brakes work fine, nothing else is working. I have checked all the fuses and switched the relays and tested no dice. Hoping it's not a melted harness. Thanks.

 

So many bad memories of boat trailer wiring! Do you have a plug tester to at least know if your truck is right? Once you prove that you need to start checking all your trailer connections... water in connections and harnesses, grounds, individual lights. It really comes down to how your trailer is wired as well. I feel for you, good luck ! ��

 

I should have mentioned I hooked the trailer to a diffrent truck and everything worked great and you can see the pins putting out the wrong voltages with a multimeter so its, unfortunately, definetly somthing on the truck.

 

You have a bad ground connection. Its anyone's guess where it actually is and it could just as easily be on the trailer even though the trailer works on other vehicles. Your symptoms are classic for a bad ground. You just need to find it.