Recently bought a boat with a 20 year old Fisher trailer that is in excellent shape. But except for the running lights the rear lights do not work. It had a big trailer socket like for brakes. This is for a 16' boat and no brakes. I believe the seller just stuck that on there to get me home as they were connected with wire nuts.

I took that off and put on a 4 pin connector and put that in the truck and now nothing comes on. Since one of the rear lights was broken I bought two new LEDs for the back and had to cut off the old ones due to rust. Looking at the wires that were attached to the right side I saw a wire that did not come out at the hitch.

The trailer in a box type and the wires are not accessible except for pulling them out. And there is three sets of running lights. Running new wire makes sense to me but hooking up the running lights thru small holes could present a problem.

Any suggestions?

The first thing to check is the ground wire. If you have a multimeter check from the insulated pin on your trailer plug to an exposed nut or bolt to make sure you have a good ground. If that checks out okay, check the non insulated pin on the tow vehicle the same way. Grounding is the most common problem. Correct any problems there first.

If the ground is good, check the HOT pins from the tow vehicle for voltage to make sure they are working as you towed it home with the large round type hooked up.

If all the above was good, hook the two connectors together and check for voltage at the new lights.
Remember, LED lights are Polarity Sensitive and you may have to reverse the wires to get them to light up.

Good luck.

Absolutely 100% on the info above, grounds are almost always the problem.

be sure the white wire is hooked solid to the frame on both for ground.

Do you want to fix it ONCE and be done with it? Buy four 25-foot pieces of 16 ga. TINNED marine wire(I get mine from boatwire.com), in white, yellow, green and brown. Get some heat-shrink tubing large enough to hold all four wires. Cut about 20 pieces one inch long and slide them over the four wires. Start at the right trailer light, slide a smaller piece of shrink tubing over the brown wire and SOLDER it to the brown taillight wire. Let it cool. Paint the joint with Liquid 'Lectric Tape. Let it dry. Slide the shrink tubing over the solder joint, and heat it with a lighter or heat gun. Same for the green wire. Put a lug on the white wire(ground). Put it on one of the light mounting studs(usually the ground). Coat that with Liquid Tape. Tie the ground to the left light ground. White: Ground. Brown: Tail/Running lights. Yellow: Left Turn/Brake light. Green: Right Turn/Brake light. I don't hook any ground wire to my trailer frame.

Repeat for all the connections, running the big shrink tubing pieces forward as you go, spacing and heating them. When you get to the front, you'll have a custom, all-soldered harness that will live. Don't run wires inside the frame, where you can't see damage. Tie-wrap the harness to the frame. Solder the four wires to the appropriate plug wires. If your tow vehicle is wired right(usually meaning a 3-bulb to 2-bulb converter), you'll have long-lasting trailer lights. It ain't rocket science. Whatever you do, don't buy a Harbor Freight trailer wire kit. I have never seen such absolute [censored] in my 65+ years of doing electrical work. Good luck! IslandJim

Bruce, I'm two doors down from you if you want help with it.

If you are going to use heat shrink tubing get some marine grade with the adhesive in it. Shrinks up to 4x original diameter.

wierd beard- I sent you a personal message. Since you are new to the TFF look at the top of the page on the right where your
name is and a little envelope will be blinking. Click on that and go to private messages.

Thank you all for your input and thanks to weird beard for fixing the problem.

Come on, you can't leave us hanging. What was the problem.
And a job well done to Weird Beard.

Someone had added wire to the trailer harness and didn’t follow wire colors. Green at the front was red at the back yellow at the front was white at the back, ect. Once we figured it out works good.

Wow, that would really mess with your head trying to figure that one out.
Glad you got it fixed and a big to Weird Beard.

Man don't you just love working on something behind someone that will do that. You know I get it, in a pinch your just trying to get to the lake, but once your back I would have replaced all that with proper color coded wire. It's not that expensive either.

Fixing that was easy in the scheme of things, it was finding the problem was the trick ...

if you ever rewire one, napa sells good 4 strand by the foot, they have round in 3-4 different wire counts.