Wiring New Trailer Tail Lights to a Trailer with Only 2 Wires

Updated 05/28/2025 | Published 03/26/2015

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Question:

Need to replace taillights on my trailer. Theres two wires that come out of the trailer for the lights. Only red and black. How do I wire those two wires for my tail lights?

asked by: Gary

Expert Reply:

It sounds like your trailer tail lights you are using now ground through the mounting hardware. That would be why there are only 2 wires. One of those two wires is going to be for turn signal/brake lights and the other is going to be for running lights.

To determine which wire is which you can plug the trailer up to a vehicle and have someone step on the brake pedal while you test the wires at the back of the trailer using a circuit tester such as # PTW2993. Whichever wire lights up the tester with the brake pedal pressed is the turn signal/brake light wire. That would make the other wire the running lights which you can test by turning on the tow vehicle headlights.

On new lights there is usually going to be a yellow wire on the driver side light for left turn/brake lights and a brown for running lights. There may or may not be a ground wire present. If there is a ground wire, usually white, attach it to a bare metal surface on the trailer frame. If there isn't a ground wire, then the light grounds through the mounting hardware.

The same thing goes for the right side of the trailer only instead of a yellow wire for turn signal and brake lights it will be a green wire.

expert reply by: Michael H

Robert M.

9/8/2024

Installing a set of optronics led lights on my small boat trailer,and I only have two wires on both lights yellow &brown, green brown,& no white wire.how do I ground the lights? Please!!il I would be grateful any information

Jesse M.

9/9/2024

@RobertM If there is no ground wire coming from the light this likely means the light is grounded via the mounting hardware. If you can tell me the model number of your lights I can verify this.

Jason B.

10/14/2024

@JesseM Hi, sorry to jump on this response. I have new trailer lights with 2 black cables and one red. One of the black cables and the red are together. And the other black is separate. Would the separate black be the ground? Thanks Jason

Jesse M.

10/15/2024

@JasonB You are probably right but it is hard to say with 100% certainty without either testing the wires or getting the model number from your light. Most other lights with these wire colors it will be black for ground and power and then red for perhaps a blinker. Wire color is not always universal though.

Craig.

5/28/2025

My boat trailer wiring harness for the lights only has 2 wires yellow brown and green black. The lights has 4 wires, how do you wire something like that. Please let me know. Thanks.

Jameson C.

5/28/2025

@Craig There should be another wire for the running lights, but the yellow is normally left stop/turn and green is right stop/turn. With that said I would still test these wires for function with a tester like # PTW2992 to determine function and then go from there.