Installing My Trailer's Ground Wire and Marker Lights

The author using his hands and a crimping tool to install the ground wire and marker light on his trailer

The next step after installing the main harness was to attach the ground wire and install the marker lights. The marker lights are the amber lights on the sides of the trailer near the front.

The marker lights on both the left and right sides of a trailer that uses a four-wire system are powered by the brown wires on the harness, which then run to the back of the trailer to power the tail lights.

The brown wires also power any other lights that are always on whenever the trailer lights are in use, such as the red clearance or marker lights toward the rear of the trailer if they're not integrated into the tail light housing.

In my case, I'd already run the brown wires through the transverse tubes and out the sides of the trailer in the previous step of my trailer-rewiring project. This step consisted of connecting the ground, connecting the amber marker lights, and running the wires aft to the tail lights.

On my trailer, the ground connection uses one of the attachment screws for the left marker light. That meant I had to lengthen the ground wire on the harness because the ground on the harness was too short, and you should never drill any holes in the trailer frame. I lengthened the wire by using a heat-shrink butt-splice connector to splice a section of wire into the harness ground before I sleeved it, so that wire was already present by the time I got to this step.

With all that wiring in place, here's how I proceeded to wire the marker lights and the ground.

The Procedure

I cut the ground wire to length, attached a heat-shrink ring terminal connector to it, and used a socket wrench to secure it to the trailer where the manufacturer did.
I ran a short length of brown wire from the brown wire on the harness using a heat-shrink butt splice connector. I cut the main brown wire and crimped one cut end to one side of the connector, and crimped the other cut end plus the short length of brown wire for the marker light to the other end.
I've found the marker light housings on this trailer to be prone to corrosion, so I decided to wire them using quick-disconnect heat-shrink terminals to make them easier to replace. As is standard electrical safety procedure, I attached the female terminal to the "hot" short brown wire that supplies the power, and the male terminal to the wire coming out of the light. That prevents the male spade connector from shorting against the chassis if it comes loose.
I applied a piece of heat-shrink tubing over the connections using a heat gun for structural rigidity, to secure the sleeving, and to protect the connections from the weather. Obviously, I'd already slipped the tubing over the wires before crimping the connectors.

That's the short version. For the rest of the story, you can watch the following video.

Video: Installing My Trailer's Ground Wire and Marker Lights

The next step in my DIY trailer-rewiring project was to run the side wiring and connect the rear trailer lights.