joe mcmillan

Those harness testers are available at Amazon. Here's a 4 and 7 pin tester for $7.99 and free shipping. For $8 spent will save hours of frustration trying to determine where the issue is.

Manuellabour247

I have a 7-to-4 adapter that I use to get a little extra length on my wiring. It always works fine, but I ran the ground wire directly to the trailer and soldered it on instead of bolting it. Made a huge difference. I use the LED trailer lights though, so they don't draw as much power as the older lights. Keeps me from blowing fuses so far.

I also put dielectric grease on my hookups and keep my trailer plug covered from the element while it sits.

homer

Those testers are awesome, I use them too as we can get them at Princess Auto here cheap. I have a couple of them. That's why I had mentioned it earlier in the post (although I posted links not pictures haha).

Your answer definitely helps, now that I know the headlamps on aren't showing the taillights, that tells me more. Since it's with multiple trailers, I'm assuming the truck otherwise I was going to suggest get the tester because it could be the trailer of course. That tester makes life VERY easy though .

I've attached the diagram for ours. Check the F10 fuse (which I'm assuming is good) and check the Trailer Tow Relay, Parking Lamps. Swap it with another if you have to (I'm not 100% sure if there's others that fit you can test with there). You should be able to swap it with your fog lamp relay if you have fog lamps. It'd be funnier if it was just missing! I've attached a diagram to show that and another to show the passenger side fuse compartment.