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Trailer Wiring and Light Replacement Demonstration

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Demonstration of a Trailer Wiring and Light Replacement

Today were going to be replacing the wiring on this trailer. We will be replacing a 4 pole flat, as you can tell this one is pretty beat up and worn, so it has to be replaced. We are going to cut this wire off and we are going to replace this brown wire also. We are going to cut this loose and pull the rest of our wire through, on this trailer here it is pretty open so we can reach the wiring. Just in case though, if your wiring was run in conduit and you are replacing it, do not pull all of this out yet. We will actually tie on the new wires and use these wires to pull them through. We are going to cut off all of this old wire here, and we are going to take off this old light too. I would like to point something out on this 4 pole flat, you will notice there are actually 5 wires coming out of it. What is happening is on the inside of this pin, which is for the running lights, has two wires coming out.

The reason for that is we can split the running light circuit to both sides of the trailer. Mainly that is used on boat trailers and other light duty ones. What is nice about that if you have a running light on the side, chances are your running wire right next to it. You can tap into your wiring without having to make a wire go all the way across. It basically provides a short cut. This case we are going to run it all the way on one side and jump over since we have a small trailer and no running lights. Now we are going to tape our new wires to the old ones and pull it through the frame. It does not take much, just enough to make it smooth so it does not snag anywhere. Now we simply pull the wire through. Before we pull everything through, we are going to make sure we have enough leader to hang out. This unit comes with a pre-wired ground wire so we are going to leave it at its maximum length and this is where we are going to ground it at and well have a good idea of how much slack we need to leave. At this point its a good idea to tape up all the wires together that way it will keep them from getting tangled with anything else. Since we have our ground here we are going to ground it but before that we are going to take this loop off for a smaller one. Since we have an existing ground that works pretty good, we will use a smaller screw. Always give it a twist when you crimp it down it will make a nice tight fit. We will re-install our screw but first we will put a pit of dielectric grease on there to help protect it a little bit, electricity still flows on the ground and the less rust the better. Now we can pull the rest of our slack out.

We have our lights pulled over to the left hand side, we are using the yellow and the brown, yellow for left turn. We need to cut this wire to length about 1/2 to 3/8 inch. The wires then get stuck inside here there is a little strip in here that pulls against it and when you pull it back it catches so it can not come out and makes electrical contact. You bottom out the plastic and give it a little bit of a pull back and it catches. Same thing with the left turn signal. If you notice there is two holes side by side here. You could actually jump another wire from this hole here to another light, so it acts like a miniature junction box. This little channel here hides the wire so when you push it up against your frame it wont get mashed. Your light gets installed just like that. Before we bolt on our light we are going to clean this area out where the holes are that way it makes a good metal to metal contact with no rust and corrosion. You can use a file or scrape it if you are in a hurry but if you can use something more abrasive. You do not have to get these super tight, just snug them down and they will be fine. Again were going to use the old wire to pull the new wire through. Now were back on the right side of the trailer and we will do the same thing, we will run the wires and install a new light. Now we will install our wiring, green for the right turn and brown for the running light. One more final touch will be to run some loom clamps to the wire to keep it up closest to the bed. It does not touch anything, but it is better to keep it out of the way. At this point we will reassemble our trailer and test out the lighting. We have our trailer wired up, lets give it a test run. Notice we did not connect our truck to the trailer so we can make sure our ground wire is working. It is not a good idea to ground through the ball because it can be intermittent from bouncing around going down the road. First we will test the running lights then the turn signals, then the brake lights. We will do this for both sides. There you have it for our install of our trailer wiring harness. We used part number A35W42B and the tail lights we used, the left hand side was part number 432400 and the right hand side was 432200.



Just put new wire on my trailer, it use to be a boat trailer made into a flat bed. When ever I turn on my head lights my break lights on the trailer will not work?

Etrailer Expert

Rachael H.


Your problem sounds like a grounding issue. Grounds usually appear when the lights are under a load, and this would be the case when your running lights (headlights) are on and then activate the brake lights. I recommend checking all the grounding locations such as: on the vehicle (wiring connection 4-way or 7-way), main ground on trailer and grounds for each light. If this is an aluminum frame trailer that may also be the issue and a ground to aluminum may not provide the correct conductivity. you might need to run jumper grounds to a steel component of the trailer (like the axle). Also if this trailer has any moving parts (fold away coupler, or tilting bed) that connection can also interrupt the ground signal. Again running jumpers to a stationary and appropriate metal portion of the trailer can help that too.



Please help I bought a used trailer I installed new five wire plug wire combination and replaced the old lights. I grounded the trailer in three places the double brown is tied to the black wire out of lights The green goes to the red the on one light. The yellow go to the red on other light both lights are grounded to trailer. The running,stop, turn lights all work one at a time,if I combine them together the turn signal is very weak what is my problem. Thanks. Rick

Patrick B.


Check the ground connection between the trailer and the vehicle. It sounds like you have a weak ground that will support flow for one function at a time, but the load of all becomes more than the ground can sustain. Check your ground coming off the new 5-way plug. Make sure it has good contact to clean metal, both corrosion and paint can act as an isolator. Sand the area so your hardware makes good contact with the steel frame, then cover with paint or a sealer to prevent corrosion. If the trailer is an aluminum frame, you probably will not be able to ground to it, you will need to run a ground wire down the whole frame to ground all of the lights.

Everett L.


I have all of the correct replacement parts unable to remove old wires they are encased in tubing will not pull out. What is holding them in? How do I remove them?

Norman V.


after rewiring the trailer when i put on the turn signal the clearance lights blink real deamly also i checked the wiring and found nothing wrong. could this be in the ground? thank you

Patrick B.


Chances are it's bleeding through the ground. Make sure the clearance lights are grounded well, make sure your trailer connector ground is on clean metal and isn't damaged or corroded, and finally check the ground on the vehicle side connector make sure it has a good, clean ground. Also, if you can take apart the trailer connector. Make sure all the terminals are clean inside and out. As the corrosion builds up inside a trailer connector, the corroded material can actually carry a weak signal and cause the bleed over.

Info for these parts were:

Employee Jacob T
Video Edited:
Jacob T
Employee Kathleen M
Video by:
Kathleen M
Employee Randy B
Installed by:
Randy B

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