After searching the forum I'm still stumped. I recently purchased a used utility trailer. I was getting irregular workings from my utility trailer lights (1157 bulbs). By this I mean only the left trailer running light worked. This evening I installed new LED trailer lights. Now nothing on the trailer functions. No tail lights, no brake lights, no hazards, no parking and no signals. The 4-pin ground under left rear of vehicle looks fine. Under the hood in the power distribution box, I checked fuses 5, 19, 20 and relay 201. All of 5, 19 and 20 looked good by close visual inspection. Fuse Relay 201 looked good (as far as I could tell). I am also installing a new "Y" wiring harness (4-flat). I am stumped. Am I overlooking anything?

I towed the boat a couple of weekends ago and all was well then. I guess I'll check the fuses in the passenger panel tomorrow. As always, Thanks in advance.

 

sounds more like a trailer wiring issue than a vehicle wiring issue. Try hooking up to a trailer with known good lights and see what happens, if all works properly than you know for sure its the trailer wiring. If you have to, start fresh on the trailer wiring, who knows what kind of wiring mess is under that trailer. I have seen all the bulbs go bad before on a trailer, and I was planning a complete rewire but I checked a bulb first and it turned out the bulbs were bad.

 

or get a trailer light tester like this one - Amazon.com: Attwood Trailer Light Inline Circuit Tester: Sports & Outdoors
There are even trailer connectors now that have the LEDs built into it.

 

90% of the time it is a ground issue. I've found running a ground wire on the trailer solves the problem. Most just use a "contact" ground, as the ball on the tow vehicle and the tounge of the trailer. There is almost always a wired ground in the wire socket on the tow vehicle, if not that's an easy fix as well.

 

Can you pin out the connecter with a multi meter? That would give you a definitive answer.

 

Problem Solved!
With this particular LED trailer light kit, Blazer C7423, I have to run two ground wires from the trailer connector (white wire) with one of the wires going back to the left trailer tail light white ground wire and the other ground wire going back to the right trailer tail light white ground wire.

Grounding each light right where they mount on the frame and grounding the white trailer connector wire to the tongue of the trailer DID NOT WORK.

 

If you have a tilt trailer I can understand it not working. You'd be relying on the hinge bolt as your connection from the bed to the frame.

 

I do indeed, have a tilt trailer.

 

Well there's your answer as to why you needed to run a wire for ground instead of using the frame.

 

I've also fought polarity issues with LED lighting. More for dome lights or imported single lights. Many of the LED's I've worked with have black and white wires (like A/C). A diode by description is a one way current flow. Some truck LED's can be wired either way. Many of the black/white wired lights, the black is hot and the white is ground.

 

Ironically that's how trailer wiring is. For some reason trailer manufacturers, as well as, aftermarket wiring harness manufacturers chose to use a white wire for ground instead of a black one.