How To Troubleshoot Trailer Lights

Updated 12/15/2025 | Published 12/12/2025

Question:

Need advise on troubleshooting trailer lights malfunction

asked by: Victor C

Expert Reply:

Hey, Victor!

Happy to help, would you be able to describe what kind of issues you are having with your trailer lights? Is it a situation where you were replacing your trailer lights and started having issues or are you have issues with existing lights on your trailer?

Let me know what goin on and we can go from there!

expert reply by: Gwendolyn Y

Victor C.

12/13/2025

I am having issue with the existing boat trailer lights. Right now, after checking and cleaning up as many of the connectors as I can see, all the lights come on except the rear center 3-lamp bar running lights. However, when I connect the power line (red) (white grounded to frame) it comes on strong but the Left and right tail lights are out. I also noticed when the running lights are on only, it is very dim. It appears not to receive full power. Where should I check next? Victor PS. I have a 4 pin/7 wire plug. Wh (heavier); Grn/Brn; Yel/Brn; Brn/ Wh.

Gwendolyn Y.

12/15/2025

@VictorC Thanks for your reply, Victor. Few more questions for you. Is this a boat trailer? Just asking because most trailers that have that 3-light bar are on boat trailers. Also, when you say you have a 4-way and 7-way are you referring to the plugs on your vehicle? Are the running lights always dim or just when your connected the red wire to your power source? If it is a boat trailer and you're having issues with your lights no matter how you're testing or sending power to the lights. It really does sound like there is an issue somewhere in the harness. I recommend getting a wishbone style 4-way like part # A35WB and rewiring the whole trailer.

Helpful Links

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