How To Power Running Light Circuit Of Trailer With 4 Way Flat Pulled By Tractor With One Light Wire

Updated 03/18/2026 | Published 03/16/2026

Question:

I would like to be able to connect my trailer lights to my tractor. My trailer is wired with the flat 4 connecter, but my tractor only has one wire run to the rear for the rear lights. Is there a way that I can use that wire to connect it to my trailer to use the main trailer lights? I realize that the turning signals and brake lights would not work.

asked by: Mervin

Expert Reply:

Hey there Mervin, there is definitely a way to power your trailer running lights by your tractor's single power wire for lights.

I'd grab something like TowSmart Trailer Connector - 4-Wire Flat Set # TS46RR (or find something locally if need be). You likely won't need the trailer 4 way end, but just use the 48" long harness with the female 4 way flat for the vehicle side. Just hardwire the tractor's single light wire to the brown wire on the vehicle end harness, then plug into your trailer's male 4 way flat conneciton.

This will power the trailer's running light circuit only. Hope this helps! Let me know if you need anything else.
Do you run the tractor and trailer at night often?

expert reply by: Andrew N

4 way vehicle side
(click to enlarge)

Mervin

3/18/2026

Thanks! No, I don't run it a lot after dark. I also use a battery-operated magnetic flashing light but thought it would also help to have my tail & running lights.