Maybe this link applies?

https://www.etrailer.com/question-56433.html

To keep from having to connect the tail lights from side to side at the
back, an extra wire runs on each side with the other wires for each side.

Mark

Jim. Felder wrote:
> I'm currently working on my trailer, re-flooring and rewiring it. I stopped
> by NAPA a while back and bought a set of two matching connectors.
>
> I put one on the car side like this:
>
> Car side new connector
> White
> Brown
> Yellow
> Green
>
> Today I set out to replace the trailer side connector with the new one. I
> discovered that the new connector has four wires, like this:
> White (ground)
> Brown
> Yellow
> Green
>
> just like the new connector I put on the car (which has a euro conversion
> brick kind of thing spliced into the tail light wires).
>
> However, the trailer, which has the Harbor Freight wiring, light and
> connector set it came with has a fifth wire, like this:
> White (ground)
> Brown with yellow stripe
> Brown with green stripe
> Yellow
> Green
>
> Can anyone tell me what is going on, and what to do about this?
>
> Thanks,
>
> Jim
>