My right trailer lights won’t work. I replaced it with a new one and that didn’t fix the problem. I checked all of the fuses on my truck and that wasn’t the problem. I recleaned the ground down to bare metal and that didn’t work. I plugged in the tester into my plug and everything is fine there. What am I missing? Any help would be greatly appreciated.
Hook a jumper wire from the truck frame to the trailer frame make sure it touching bare to bare metal, just to double check the ground.
Do any lights work?
If the ground check doesn’t fix it you could have an open wire anywhere from the trailer plug to your new light. If you have a voltmeter or test light make sure you have 12V coming OUT of the trailer side of the plug and work your way back to the light. Also check the right light ground wire.
Did you replace just the bulb or the whole fixture? If it was just the bulb the socket could be corroded (bad ground or 12V connection).
A vexilar battery is handy when troubleshooting trailer lights….rules out the truck.
Most of the late model trucks with the towing pkg. have a seperate fuse for the trailer lights. GM for sure.
Did you replace the light bulb or the whole light assembly? When you say the right lights don’t work, do you mean the running lights, the turn signal/brake lights, or both? If it’s both, then you’re missing a ground connection somewhere between the light assembly and the connector. The light assembly typically makes connection through the mounting bolts to the frame of the trailer. Make sure you’re bolting down to clean shiny metal. If there’s a problem in between, you can diagnose this by temporarily hooking a piece of wire between the trailer light connector ground and the light assembly ground.
You can also check out:
Rootski
Do a continuity test on the appropriate color wire(s) for the light that does not work. By this I mean just take an electrical tester and a spare piece of wire (long then the trailer) and connect to both ends and verify that you have continuity through that particular wire. I would be willing to bet that you don’t. Pull a new wire and splice at both ends (marine grade) and you are back in business 
Good Luck
cheers
Had the same problem with mine, Have a newer Doge Ram and my brother-in-law has a Ford F150, and we found out that there is a seperate fuze box under the hood that has fuzes for the Trailer tail lights, it was driving us nuts until we read the book 
, good luck
I have a 99 F-350 and there is a fuse under the hood in mine as well. It was blown….replaced it and it fixed the problem.
JB
The left side brake, turning, and running lights work. nothing on the right side works. I’m going to try a few of the suggested remedies right now. Thanks for all of the input. I’ll let you know what happens in the next couple hours. 
Ended up having to splice a new wire. Lights work great now. Thanks everyone for your help!
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