How I Rewired my Utility Trailer Lights

Close up of an operational right tail light on the author's utility trailer after replacing the wiring

Back at the beginning of the COVID-19 pandemic, I decided to rewire the lights in my utility trailer. The wiring needed replacing, and I had time to kill.

My trailer uses the standard four-wire lighting system used in most light utility and boat trailers in the United States, so I was able to use a standard trailer wiring harness.

I also decided to take some steps to improve the reliability of the trailer wiring, however, by protecting it from damage due to corrosion, vobration, physical hazards (rocks and dirt, for example), UV light, and moisture.

You see, trailer wiring is notoriously prone to failure due to vibration and the environment. So I spent a bit of time thinking about these factors and how to best the trailer's wiring harness from UV light, vibration, abrasion, short circuits, and moisture. Two annual state inspections later, the lights still work; so I guess I did a pretty good job.

The first thing I considered was vibration. Vibration causes the trailer wires to rattle against the metal chassis of the trailer and to rub against each other, which can cause the insulation to chafe over time. That in turn can cause malfunctions and short circuits.

There was nothing I could do to prevent the trailer from vibrating, of course; but there were things I could do to protect the trailer's wiring from being damaged.

The first thing I decided to do was abandon my original plan of building a trailer wiring harness myself out of separate wires. Rather, I bought a Y-harness with the wires for each side of the trailer bonded to each other. Wires that are bonded to each other move as a unit, so they can't rub against each other and chafe.

Male commercial drone pilot flying a drone at a construction site.

The second thing I decided to do was encase as much of the wiring as possible to protect it from chafing, UV light, and debris kicked up from the road. After considering the options, I decided to use expandable braided sleeving. Other options would have included split wire loom or spiral wire loom. I chose the braided sleeving because it would be easier to pass through the metal tubes the trailer manufacturer provided to run the wires through at some portions of the trailer's chassis.

My third guiding principle had to do with moisture. Most trailer wiring failures occur at splices where moisture corrodes the connections, so I decided to use as few of them as possible. I also decided to use heat-shrink connectors, which have adhesive inside the insulation that helps produce weather-tight seals, for all splices and terminal connections. That would help protect the trailer wiring's connectors from moisture and corrosion.

I also made one concession related to ease of repair, and that had to do with the OEM marker lights. I've gone through quite a few of them. The housings seem to be prone to premature failure from corrosion. So rather than splicing or hard-wiring the marker lights into the system permanently, I decided to use heat-shrink quick-disconnect terminals for those connections to make it easier to replace the lights in the future. That way I won't have to cut into the wiring to replace a part that my experience has proven to be prone to failure. I may also try to fabricate a gasket to put between the lens and the housing to keep moisture out.

None of this is earth-shattering, mind you. I didn't invent any new technology or methods, and NASA hasn't called me about designing the spacecraft for the first manned mission to mars. Yet.

All in all, though, I'm very happy with how the project turned out. Other than replacing the marker lights, which for whatever reason are especially prone to failure, I'm hoping this will be the last time I ever have to rewire this trailer. It's been three years since I did the trailer-rewiring job so far, and it's still going string.

About This Site

The main reason I decided to build a Web site around my trailer-rewiring project was because it's what I do. I'm a semi-retired Web developer. I also had time to kill during the COVID-19 pandemic of 2020. I sincerely hope it's over by the time you read these words.

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The site actually took a lot longer to code and write than the trailer did to rewire, what with editing the videos and so forth, so I'm glad you're reading it. The actual time I spent rewiring the trailer totaled maybe three or four hours, if that much; but editing the videos and building the site took three or four weeks.

That's okay. Like I said, part of why I built the site was to pass time during the pandemic.

Most of the videos on this site were taken with an older GoPro Hero action camera. I'm told the newer ones have better video quality. A few segments of the videos and most of the still pictures were taken with a OnePlus 7t phone. The videos were edited using a homebuilt dedicated video-editing computer and Magix Video Edit Pro software. The site itself was hand-coded on another homebuilt computer. The markup uses HTML5 and CSS3, and the scripting language is PHP. There's also a bit of JavaScript here and there.

In a nutshell, this Web site is a chronicle of a few days in my life that I spent planning and carrying out a complete trailer-rewiring project, in part to alleviate boredom. It's not really a "how-to" site. I'm sure you can find better tutorials about how to wire a trailer. Rather, it's more of a "what I did" site. It's the story of how I spent a few hours of my life. But if it inspires you to rewire your utility or boat trailer in a more reliable way, I'll be happy to have helped.

Important Safety Precautions when Rewiring a Trailer

Just in case the last sentence of the previous paragraph describes why you wound up on my site, my lawyer urges you to read and follow these important safety precautions:

If you have to work under your trailer, always use jack stands. Never work under anything while it's being held up by only one support or by a hydraulic jack.
Always use wheel chocks in front of and behind all wheels when working under a trailer (or anything else, for that matter). This is especially important when working under a trailer that has no brakes.
As a general rule, never drill holes in the frame of a trailer. The structural members of a trailer are subject to enormous physical stresses while in use, and even a small hole in the wrong place might cause the metal to fail. That's why trailer manufacturers typically mount the lights on separate tabs welded to the frame rather than to the frame itself.
Heat guns are, well, hot. Be careful when using a heat gun or any other source of heat to shrink insulation or tubing. Also, it may take several minutes for the metal nozzle of a heat gun to cool down after use, so don't grab it there. (Don't ask me how I know...)
Properly-functioning trailer lights are mandatory when using a trailer on public roads. Make sure to check that they are functioning properly before every trip, and every time you stop during a trip.

I hope you enjoy my site. Thanks for stopping by.