Went to use my trailer last week and no lights. Truck has factory 7 way plug have had zero light problems since new.

I checked all the fuses and relays in distribution block and nothing bad.
Pulled the factory plug off the back of 7 way and its not rotted, Checked the wires themselves with a test light and now power for anything. Found a ground wire with ring terminal rotted off near hitch repaired that still nothing.

At my whits end, no brake lights, running lights or turn signals at plug with test lamp. Truck is a rust belt truck.

All lights on truck work.

 

Are the pins rusty?

I find that every spring when I go to tow the trailer I have to clean up the pins in the plug or I get nothing. I was going to pack it up with di-electric grease this winter but forgot so I am going to be cleaning the pins up again this spring. Thanks for reminding me, now I will do it before I need to hook up the trailer.

Never mind I see now you checked the wires themselves and have no power. I would pull the fuse and replace it even if it looks OK and see if that works.

 

So if you turn on the running lamps and hazard flashers, then ground the test light and probe the pins, the test light doesn't illuminate? Have you confirmed the test light is good?

 

From your description (nothing works) I'd say you still have a faulty ground. Your description of your truck as "rust belt" suggests to me that the frame ground path back to the battery is bad.

I'd run a jumper wire from the ground pin on the plug all the way back to the negative battery terminal. Then, retest.

- Jack