Ok, so slightly OT, but hopefully you guys can help me here. I am selling one of my horse trailers and I had a brake light out--went to replace it and decided to change out the whole light because the wires in it were rusty. So I spliced a new one in (Red, Black, Ground). I wired it correctly. When I tested it---It worked, but when I put the headlights on (which turns the running lights on for the trailer---that one brake light didn't work, but the other did. So I crossed the wires thinking maybe the original manuf. wired it wrong. Same thing happened--nothing different. So I put the wires back to red on red and black on black and tried to see if the6 pin plug was dirty-poked a round with a toothpick in there. It didn't seem too dirty and it hasnt rained in months (I live in CA). So my electrician neighbor said--"this may make it worse or fix it but try spraying a little WD40 in the 5 pin plug and hooking it up and seeing what happens". So I did that and now--It doesn't work at all even when the headlights are off. The other brake light still works though--which is weird. Do you think its not grounding? I don't get it.
Each pin in the plug corresponds to a different function ground, running lights, left turn light/brake light, right turn light/brake light, brake controller, power. You think the one pin is bad?

 

I would first start by determining if the circuit is good between the trailor's connector and the light. I would suggest using a digital voltmeter to check it with.

1) Disconnect the trailor from the truck.
2) Remove the bulbs in the trailor light housings and test for continuity between the hot wire and ground on the trailor side of the connector. There shouldn't be any.
3) Replace all of the bulbs and recheck for continuity. There should be continuity between hot and ground on all of the circuits now.

If this proceedure doesn't solve it, you could use a 12vdc source dirctly to the trailor plug to verify that the circuitry is good between it and the light, testing it individually. Just make sure you connect it with the correct polarity to keep things simple. If it lights up by doing that, then the issue is somewhere on the truck it is hooked up to. If it doesn't work, then you know that the issue is in the trailor wiring. Hopefully this is the case because the wiring should be easier to trace on the trailor. Just work your way back from the connector to the light that doesn't work and verify that there are no broken areas in the wire, and also look for any bare spots in the wire while you're at it.(These should have been found while testing with the meter and by using the 12vdc source, but it's still a good idea for making sure there aren't any present, possibly causing future issues.) Just because the insulation is still intact doesn't mean that the wire is still goog. You'll be able to tell if the wire is borken by the way it feels. The wire should bend the same way throughout the entir length. If there is a spot that feels different, usually where the wire makes a sharp turn in the framing or where it may be subject to being pinched, then it may have broken inside of the insulation. If this is the case, you would just properly splice the wire at that point.

Your electrician friend should be able to help you with what I've suggested. He should have a digital voltmeter as well.

 

Most of the time I have had issues it usually is due to a bad ground. I use a 12 volt test light, first I check at the vehicle plug in and verify voltage, then go to the trailer and verify voltage with the test light, if my light doesnt come on when testing I try changing the location that I have clipped my tester to. Sometimes you can get voltage in the socket but if the light assembly is not grounded good then the bulb wont come on even if the socket is hot. Try checking the bolts that hold the light assembly on the trailer, make sure they are clean and rust free. Another way you can check for a bad ground at the light assembly is to have power going to it from either having the blinker on or the brake pedal pushed down and use to test light with its clip hooked up to a good ground on the frame and touch the side of the bulb holder inside the assembly with the bulb still in it and if it starts working then you have a ground problem. I hope this helps and I feel your pain from past light problems myself!