Had the same problem years ago. The lights were not grounded.

Sounds like a ground problem. Make absolutely sure your ground wire to the trailer is touching raw metal surface of your trailer tongue. Remove the grounding bolt, sand, steel brush, etc. the surface area the grounding bolt will be placed. Reapply the grounding bolt tightly, making sure enough of the grounding wire of the trailer is stripped of insulation so it is making enough contact with the grounding bolt. If this does not work you need to get an electrical meter and start troubleshooting from your tow vehicle to the trailer.

When I installed the new harness last night, I put a new eyelet on the ground wire, the wire was stripped well and it's attached to a nut/bolt on my trailer very tightly, however if the trailer coupler was not placed on my hitch/ball would this cause it not to ground when I tested the lights? I will look into it more tonight. Thanks.

This is what I did when changing the trailer lights. I run a separate ground wire from each light straight to the front and then ground it to the tongue in an area that wont get submerged. Solved all of my previous lighting problems.

Definatly connect the trailer to the truck and try!happened to me

When I wired my trailer, I ran the white wire straight through. No grounding through the trailer.

If you've connected the ground wire properly through the trailer light connector, and properly grounded it to the trailer then the trailer does NOT have to be on the ball to get ground. In fact it should not depend on getting the ground that way. Get an electrical tester (multi meter) and start checking the ground connections, etc. Trailer lights are very rarely complicated, it is usually a matter of something not making a good connection.

Appreciate it as I'm not to handy when it comes to electrical issues. I picked up a new connector for my vehichle, the 6 prong to 4/5 way plug as my old one had the male post break off so that might be the problem. Also I decided to get a new set of lights at $6.99/light b/c the old ones are very old. I had LEDs but they did not last, so I put the old lights back on. I only use this trailer to launch the boat a couple times a year as it is for my duck rig which stays at the camp in the water during duck season so el cheapo lights will work just fine if I can get the dang things to work. Thanks again.

The new plug connector works, new harness installed and grounded properly (old harness had a cut in the wires and too many splices), and old light kits. Brake lights work when the truck lights are not on, but when my truck lights are on and I tap the brake the trailer lights go out. Maybe I have the wrong bulb replacements? I think it's easier at $6.99/per light kit to just go ahead replace them too versus $2.99 for just the bulb.

All you need to do is disconnect your ground from your trailer and run it to one of your tail lights this will solve your problems i had the same exact problem just the other day chased my tail for three days before i realized how to fix it all i did was ran my ground to the tail light and they work fine now

If you are having a problem with grounding that is often the best fix -- run a dedicated ground wire and not depend on the trailer frame.

i learned a long time ago to just go to home depot and buy a $7 fifty foot long extension cord and wire your trailer with it. (14 or 16 guage is all you need)

its heavier wire (most trailer wires are 18 or 20 guage) and they are well protected from sunlight and abrassions by the rubberized casing so its not exposed to dry rot and cracking and you get a pre-run ground wire to each light.

color codes are easy

green is ground
white is for lights
black is for brake and blinkers

makes it hard to go wrong

i just buy the trailer to truck plugs as needed.