Utility trailers need lights---brake lights, running lights and signal lights---for safety reasons. It is also against the law in many states to tow a trailer with no lights. Marker lights should be placed every 3 feet along the sides and back of the trailer. Any local auto parts store will carry trailer combination lights. The lights are a combination (as opposed to separate lights) of the brake lights, signal lights and tail lights.
Mark the locations of the combination lights and the marker lights on the trailer. Make a small hole in the center of the spot they are to be mounted for the wires to pass through into the trailer. Bolt the tail lights and the marker lights to the trailer.
Access the wiring harness next to the tail lights on the left side of the tow vehicle. Use a circuit tester for this procedure. Turn the lights on and check the wires at the tail light to find the wires that have power. Start the engine and turn the turn signal to the left. Check the wires for the one that is flashing, and mark it "Left." Do the same for the right side, marking it "Right." Wedge or have some press on the brake, and test the remaining wires for the one that has power with the brake depressed. Mark this one "Brake."
Make sure the harness connector has enough length to reach outside the vehicle. If it is a truck, it is easy, but if it is a car, you will have to find a location to have the wires enter the vehicle from the bottom--usually where the license plate wires exit to the light for the plate. Insert the wires from the outside in. Attach the three wires of the harness to the three wires marked with the quick splice connectors. The fourth wire (black wire) is the ground, so connect it to a good ground on the frame of the tow vehicle. Mark the wires at the outside connector so that the wires can be identified.
Connect the end of the roll of 14-gauge wire to the first wire on the connector with a butt connector. Roll the wire out to the left side tail light and over to the right side with enough length not to be too tight (leave about 2 to 3 feet extra). Cut the wire. Cut three more pieces the same length.
Run all four lines under the trailer, and tape them or use wire ties every few inches to keep them together. Attach the bundle to the trailer with plastic wire holders.
Remove the insulation from the left-side tail light wire. Do the same on the wires on the trailer's combination lights. Keep the other wires from contacting any metal or each other. Attach the left-side tail light wire to the combination light by twisting the wire temporarily. Attach the black ground wire from the tow vehicle permanently with the quick splice. Run the remainder of the wire under the back of the trailer and connect it to the ground on the other combination light with a butt connector.
What you're looking for is the right wire on the tail light for the brake and the one for the tail light. The tail light is the tall element in the bulb, and the brake light is the low element. Turn on the tail lights and see which element is lit. If it is very bright, it is the wrong element. The brake light is the brightest. When you find the right wire, go to Step 7.
Attach the tail light and the right turn signal to that side with a butt connector. Do the same for the other side. Install the right-side turn signal and tail light to the same color wire. Install the brake light wire using a quick splice connector, and continue to the other side where a butt connector will be used. This is so that if one brake light goes out, the other will still operate.
Install the wiring for the marker lights. Start on the right side front marker light. Attach the wire to the positive marker lamp wire with a quick splice connector. Move on to the rear marker, and do the same, and then down the side. Make a small hole for it to go through the floor and attach it to the right-side tail light wire. Do the same to the left side. When the turn signals are on, not only will that side turn signal will be on, but the same side marker lights will flash which enhances safety.
