I'm having an issue with my trailer light while it's hooked to my f150 the left side brake lights want work on the trailer but the running lights work fine. I know it's something with the truck because when I use the same plug and trailer on my z71 everything works flawlessly. Anyone else with this issue or that can shed some light on this issue for me? Thanks in advance!

 

Find the pin out for your trucks trailer plug. When you've identified the terminal(s) for the left side brake light/turn signal, put a multimeter on VDC, then between that terminal and ground on the truck. It should be zero (or close to it). Then have someone apply the brakes. It should now show close to 12v. If it does not it could be one of a few problems.

Most probable is a loose/broken wire inside the cord cap. The wire could also be corroded somewhere between the power distribution center and the cord cap.

Another possibility is part of your power distribution center has failed..

Hope this helps a bit.