How to Rewire a Boat Trailer Like a Boss

So, the trailer for your personal watercraft has a light out. It happens to the best sailors and fishers, and it isn’t always an easy fix. That’s what this guide is here for. From troubleshooting to solving the issue, here’s everything you need to know.

The Cause

You may not want to hear it, but there are only two reasons the lights on your trailer keep going out. First, there’s a loose connection. That’s an easy fix. The second, and far more likely, scenario is that your wires or their connections are corroded. Unfortunately, that’s one thing having quality insurance does not cover.

Corrosion is bound to happen from regular wear and tear. If your trailer is more than five years old, then you can bet your top dollar that you need to replace the wire harness. This is a four to five-hour project, but it doesn’t take much skill to complete. It certainly isn’t as complicated as rewiring used outdoor boat motors. Here’s a step-by-step on how to rewire a boat trailer. 

Step 1: Pick Out Your Harness

Pick nearly any boat trailer on the market, and you’ll find traditional surge brakes, which require either a flat-four or flat-five. The four is the traditional of the two, containing five colored wires. The five has an extra blue wire used to deactivate your tailor’s surge actuator, which comes in handy for disc brakes.

Knowing which one you need is simple. Just check to see which kind your trailer had to begin with, then purchase that model. Neither is better than the other. They just function differently for separate trailer builds. Both online shops and marine dealers sell each kind.

Step 2: Run the Wires Through

Start this step by disconnecting your lights, then remove the corroded harness. Take a long, hard look at the wires and remember how they remained secured in your trailer. They might run through a tube-shaped frame or simply sit within the frame itself.

Each style of harness has two sets of wires. The brown and yellow set go along the left side, while the brown and green set goes to the right. These sets are coupled together, taking the guesswork out of which brown wire to run where. As for the white wire, connect it any bolt of your choosing behind the coupler.

There’s also a blue wire on the flat-five harness. You’ll often find similar setups inside of G3 Boats for their lighting. This connects to the surge actuator via a solenoid. Here’s how to rewire a boat trailer the easy way: Use the old wires to run the new ones through.

Step 3: The Running Lights

This is where you can help prevent future corrosion from saltwater. Instead of relying on the pinch-wire connectors (the ones cutting through the insulation towards the 12-volt pigtail), use crimp connectors. Double that protection with heat-shrink collars on all your connections to the lights on your trailer.

When connecting, you need to find the ground for the ground wire. You might find it on a connecting bolt or a separate white wire. Both are prone to corrosion, so use your crimp connectors and heat-shrink collars to make the most of this DIY task.

Step 3: The Brake and Turning Lights

Unlike your running lights, tail lights have two circuits. One is for their running light, and the other is your brake and turn signals. The running red lights are indicated by either a black or brown wire, making them easy to find.

For the brake and turn signals, each side is color coded. Red or green usually indicates the right, while yellow is on the left. Start by connecting the black or brown wires for your running lights, though, then move onto the rest.

Connect the yellow wire on the left of your harness to the yellow or red trailer wire on your left. Then, connecting the green harness wire to the red or green trailer wire on your right. Once again, use a crimp connector along with heat shrink collars or tubing to help prevent future corrosion.

Once you’ve completed this step, it’s time to check the connection. Hook your trailer up, start your car, and identify whether all of the lights are functioning properly. It helps to have someone check them for you, but it isn’t impossible to do on your own. If all the lights are working, move onto step 4. If not, then go back to the step about your broken light and repeat.

Step 4: Secure Your Work

Finally, it’s time to secure your wires in place. You might not have to do this if your trailer has tubing for the wires, but C and I-beam frames need galvanized clips to hold their wiring in place. You can pick these up along with your harness for relatively cheap.

The trick here is to make sure your wires are secured to the frame without pinching them. The clips you buy will bend slightly, allowing you to find the perfect amount of pressure. Don’t be afraid to play around with them until you secure a solid fit.

Do not, under any circumstances, skip this step. Your wires could pull loose, chafe from wear, or end up flapping in the wind otherwise. All of these are potentially hazardous situations when you place them in water to retrieve your boat. Plus, no one wants a ticket from a malfunctioning trailer tail light.

Step 5: The Final Test

Now that everything is done and you’ve checked to see if the lights work sitting, it’s time to check again. Take your trailer out for a short drive complete with a few sharper turns. If everything stays connected and lit, then you’re done. If not, it’s back to the drawing board to find out why a wire came loose.

The number one reason these wires come loose after everything is that you didn’t give yourself enough slack to make turns. This is easily fixed by moving the clamps and providing more. If not, you might have to start all over again with another harness.

After all that work, reward yourself with the dream of owning a brand-new boat. Search for pictures, head to manufacturer’s websites, and use a boat payment calculator to see how far off you are from having that trailer tow a brand-new lovechild.