For the first time the other day I went to hook up my buddy's trailer to use it, well when I hooked up the wiring to the factory wiring no lights worked on the trailer but when I would turn the blinker on the right blinker would work on the truck but the left blinker on the truck would blink too just dimly - keep in mind nothing is working on the trailer. Lights on trailer do work when connected to his truck. What in the world? ( bought the truck used. 04 FX4 )

Please help

 

I'm no expert but most all trailer lighting issues are caused by a bad ground. That's where I'd start.

 

is this a 4 pin or 6-7 pin plug?
also maybe he has wired his truck and trailer to match....

i have seen people mess up wiring..

green is right-signal/brake
yellow is left signal/brake
brown is park/running
white is ground..

with it plugged in to your truck, it seems your back feeding/shorting the wrong curcuit with the right signal causing the left to light dim..your feeding the left lights to ground..more lights-more resistance...

 

It is a 7 pin and then a 4 pin to the left of that. It's that factory plug in. No cutting or broken wires anywhere I can see

 

I would suspect a bad ground. Is the correct turn light on the trailer flashing in sync with the turn signal on the truck or is it on and when the truck's turn light is off. If so it is a bad ground. Most utility trailer lights ground through one of the mounting bolts and they always corrode. Try getting a wrench loosing the nuts and retighten, otherwise remove the ight clean off the rust an remount (and/or run a ground wire to the tongue) also check the ground wire at the tongue where it is bolted to the trailer chassis for rust build-up.

 

I have a 2013 f150 with tow package and when I tow boat with 7 pin it blew 2 fuses took to dealer and found that the tow trailer relay fuse + another fuse was not installed which they installed and replaced the blown fuses. We never had an issue with the 4 pin. Now that tow relay fuse installed if i use 7 pin will it blow fuses again?