How to Wire Trailer Lights




Introduction: How to Wire Trailer Lights

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Video tutorial on how to wire trailer lights. So here I have an older trailer which needed some work, along with re-wiring the lights. This is a fairly simple setup, no license plate light, or clearance lights. Be sure to check with your local laws in order to determine what is required for a lights. I’ve provide three different wiring diagrams at the end of the video for a better understanding. To make this tutorial easier to understand, I will be flipping the trailer on it’s side so everything is easily viewable. This also makes it much easier to work on, so if you have a smaller trailer and can do this, I would definitely recommend it.

Tools/Supplies Needed:

  • 4 wire trailer light harness
  • new lights
  • wire cutters
  • wire strippers
  • split loop poly casing
  • electrical tape
  • soldering iron
  • adhesive filled shrink tube
  • drill
  • center drill and drill bits
  • center punch
  • hammer
  • paint marker
  • retaining loops
  • cable ties

Step 1: How to Wire Trailer Lights

I have already removed what was left of the old wiring and will be installing everything new except lights. LED lights had been installed on this trailer about a year ago. If any paint work or welding is required, I would recommend doing that first before installing the new wiring.

To make this tutorial easier to understand, I will be flipping the trailer on it’s side so everything is easily viewable. This also makes it much easier to work on, so if you have a smaller trailer and can do this, I would definitely recommend it.

Step 2:

Here is the brand new generic wiring harness. This is just a standard 4 way connector with a ground, parking, and signal or brake lights. I believe there is about 25ft of wiring which should be plenty for a trailer of this size. They also provide the vehicle side connector too which I don’t need as the tow vehicle already has that installed. As you can see on the back of the package, they typically provide a guide so if you need extra help, this is a great reference guide.

Next I have 1/4” diameter, 25ft in length poly split loom casing to protect the wiring. This is a must considering the wiring will be on the underside of the trailer and is exposed to moisture, gravel, maybe salt from winter, road grim, and anything else that could damage the insulation on the wiring.

In order to hold the wiring in place, cable ties can be used but I’d rather have something a little more durable and cleaner.

Step 3:

You’ll need to plan where to run the wiring, considering this is a tilt trailer I’ll run it up the center and then it’ll split at the back of the trailer to each light. Try to keep it away from the wheels as that’s where most of the road debris will be thrown up.

At the tongue, I’ll run it on the top to keep it away from any hazards and it’s centered depending on which side the vehicle’s trailer plug comes from. On top also helps keep it away from the ground and scrubbing on the road.

I measured out the hole, used a scriber just to lightly mark the paint and not the steel. Be careful when using a scriber as this can create a spot for steel to crack, so it’s normally not recommend on structural components.

Use a center punch to prevent the drill bit from wandering.

First starting out with a center drill and then finish up with the correct sized drill bit.

Do the same for the other holes too. I’m trying to keep the wire clips somewhat closer together to prevent the wire from being caught up on anything. It’ll sit inside the channel too, so it helps add protection too. If you have a tubing frame, you may consider running the wire inside of that. Being that this is a structural piece, small holes won’t really affect the integrity too much. When you start cutting big holes in a components, that’s when the structure becomes weaker.

After I can run the wiring along the side of the tongue as the top of the tongue will be against the bed. The wiring will also need to clear the lock for the tilt.

Right now I am using a paint marker so it’s a bit easier to see, once the wiring gets to the end of the bed, it’ll be y’d off to each side.

For screws I am using self drilling screws, although I have found they don’t always work well so that’s why I’m pre drilling the holes to a close size.

While I’m drilling, I’ll also mark the holes for the wiring which will run through the channeling in the rear. Use a center drill as a pilot hole and then select the correct size for the split casing. Make sure to de-burr these holes so you don’t risk damaging the casing when pulling it through. Just use a larger sized drill bit or step drill and turn it by hand.

Step 4:

Now tape the trailer wiring together is about 16” spacing, this just helps keep everything organized and makes it easier to insert in the split casing. Do this for the full length until it y’s off at the end of the bed.

Insert the wiring into the casing, for the length of the tongue and bed. At the y’d portion, this will continue to one light, trim off the access, and the remaining section will be using on the opposite side.

For the ground wire, I had to extend this. The lights are connected to the bed, but the tongue is a movable piece which may have grounding issues at the pivot point. So the ground wire needs to be attached to the bed. If you do not have a tilt trailer, then connect the ground wire to the tongue. I soldered the extension, used adhesive filled shrink tube to protect it from moisture and matched the existing 18 ga. wire.

Seal up the split casing at the plug using electrical tape. This will keep debris out of the casing and prevent the wiring from popping out.

Beyond that, also apply electrical tape strips just like we did with the wiring around portions of the split casing too. This keeping the wiring inside and helps prevent dirt from getting inside.

Step 5:

Moving onto installation, use those plastic loop clips with screws and install the wiring into place. For length, have the plug extend a couple inches past the end of the hitch so it has a bit of movement when turning, but not tight where it can put strain on the wiring, drag on the ground, or become unplugged. This can be adjusted according to the length of your vehicle’s harness length too.

Careful when tightening up these screws, they can twist the loop, so take your time with power tools or use a screwdriver for tightening.

These loops hold the wiring snug, so I was able to leave access wire on the tilt lock so there’s no interference. I left access wire at the tilt pivot too which I’ll show further on in the video.

Once at the end of the bed, insert the wire in the casing, this will be ran to the driver’s side. The green and brown wire goes to the passenger or right side of the vehicle and the yellow and brown goes to the driver or left side of the vehicle.

Trim off the access wire and casing, remove the wire from the casing and then use the casing for the opposite side.

Feed the exposed wire into the casing, this will go against the already installed casing and taped up after holding everything together.

Feed the wire into the drill holes, on the outside skirt I drill a larger hole using the step drill so it can accommodate a rubber grommet. It looks cleaner and holds onto the casing firmly. For this I will be soldering the wires instead, you can use weatherproof connectors too, your choice. As mentioned before, this trailer has LED lights installed.

If you are using existing lights and the wires are dirty, they will need to be cleaned in order to have a sufficient connection and for the solder to stick. I used adhesive filled shrink tube as well for a waterproof seal. Again the wires are taped up with the casing. Yellow is for the left turn signal and brake light and green is for the right turn signal and brake light. I’ll included a diagram at the end and add if you want to install clearance lights too. When installing your lights, you may need to run a test and refer to their diagram. Turn signals and brake lights should be the brighter illumination and parking or clearance lights should be the dimmer illumination.

Now for the wire and casing, it has been cut longer, especially the casing since there is a few inches of wire coming from the lights. So the casing will help protect the light pigtails too. The reason for the longer wire is that if anything were need to be adjusted, we have an extra 6 inches easily for any modifications. Perhaps you want to add clearance lights, extended the plug length at the tongues end, change out the lights or whatever else may need more wires. Wrap electrical tape segments around this split loop casing too just like before.

Step 6:

Just to give you a peak before I flip over the trailer. You can see the wire is fully cased in poly split loop, have a sufficient amount of loops to hold the wiring in place and prevent it from hanging down. On the back side, I have made it a few inches away from the back skirt to reduce the chance from being gather in mud build up or if a hook from a strap is connected to the bed, it won’t clip on the wire accidentally. There is enough access wire at the tilt pivot too. I still have the ground wire installation left.

To install the ground, this will go on the bed directly to the frame member. Using a self drill screw with a pre-drilled hole, ensure there is enough wire if you are working with a tilt trailer. Install an electrical contact loop and the mounting surface may need to be cleaned. You can even apply some liquid tape to the fastened point and around the wire to prevent any moisture penetration.

Now to flip the trailer on it’s wheels again, I need to do the opposite side that was sitting on the ground.

Step 7:

Drill the hole for the grommet using the step drill.

Install the grommet and pull through the wire.

Strip the wires, solder the connection and add shrink tube.

Each light has it’s own ground shown by the white wire that connects directly to the bed on the light brackets. Again closing the split loop end with electrical tape.

Step 8:

And finally we’re down to testing, ensuring everything works as it should. Plug it into your vehicle, ensure the brake lights, four way flashers, parking lights, signal lights are working correction. If you are having issues, you may have a bad ground, check the condition of the connector and wire at the vehicle or you may have a bad power wire connection.

Step 9:

As for the wiring diagrams, here is a couple different versions to go with. I’ve based it off this wiring setup where it runs up the center and branches out at the end of the bed vs running it on each side.

First is the setup I have done.

Next is wiring up a license plate light if you chose that option. The license plate light would run off the parking lights, known as the brown wire so you can pull the power supply from either side.

And the third diagram using clearance lights. One clearance light on each side and then one larger one at the back. These too would run off the brown or parking light wiring, same as the license plate light. If you want to install a license plate light, then just run another extension from the clearance lights.

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    5 Discussions


    Question 8 months ago

    followed directions, double checked to make sure, but blinkers don't blink when trailer conected, without trailers work just fine?


    2 years ago

    Kudos on a quality job. I like your use of grommets, soldering (instead of crimped connections, yuck!), and adhesive lined heat shrink tubing. On my boat I take it 1 step further and before I slip on the tubing, I smear some dielectric grease on the soldered joint, then shrink the tube.


    Reply 2 years ago

    Thank you for the kind words and sharing your tips as well :)


    2 years ago

    I applaud you in some respect and have some issue with others. You did a good tutorial about wiring trailer lights and even did a pretty nice install. A couple things I would like to suggest though. First, you used a nice grommet at only the side of the trailer for the cased wires to run trough. But you didn't use a grommet at other locations that are just as prone to chaffing through he casing and insulation and short. It won't happen initially, but it will when you least want it to. Been there, done that.

    Second, the typical consensus on most boating sights is to run the white (ground) wire to every light and connect for their ground. Grounding to the trailer will certainly cause issue down the road. The ground on the trailer will corrode in time and the ground will be iffy then. And that is because of dissimilar metal corrosion. If you run individual white ground wires to all the lights and tie them together, you will never have a ground issue. Again, been there and done that.

    Other then that, I like your video and casing idea. I have wired lots of trailers and I like using the split loom myself. Thumbs Up!


    Reply 2 years ago

    Great to hear and thank you for the input.