Which Wires Are Used When Installing Marker Lights On Trailer

Updated 07/16/2020 | Published 04/08/2015

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35 Ft 4-Way Trailer Wiring Harness - Wishbone Style - 42" Ground

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Hook Probe Circuit Tester

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Question:

Hi, It is clear to me that there are 2 wires for left rear and two wires for right rear. My question is the marker side light wiring. There is only 1 wire coming from the marker, should I tap into the same light running to the rear right and left? same side for each respectively? If so, which wire should I tapninto for the markers? Thanks!!

asked by: Anton N

Helpful Expert Reply:

For a typical trailer wiring you will have a brown and green wire for the right-hand side of the trailer and brown and yellow wire for the left side.

If this is a new wiring installation and you are installing wiring like part # A35W42B, the brown wire that runs down each side of the trailer is for the marker lights and taillights. The yellow and green wires are for the left and right turn signals. You will want to tap into the brown wire for the marker lights.

If you are adding marker lights to an existing system, you can use a circuit tester like part # PTW2993 to test the wires for function when it is connected to a vehicle or a 12V power source like a battery.

I have attached a helpful article that explains trailer wiring and what the typical functions are.

expert reply by: Jeffrey L

Bill A.

7/12/2020

I have 2 wire side marker lights. Both wire are same color . How do yiu know which is which?

Chris R.

7/16/2020

I'm assuming one is for power and one is for ground, and if they're the same color it's POSSIBLE that either could be used for either purpose. If there isn't any label on the light housing that distinguishes this then your best option is to actually apply 12V power to one to see what it illuminates.