Electrical issue

Most trailer wiring is plugged into the harness at the left rear tail-light. Sometimes it's easily seen from under the truck or by removing the left tail-light. I'd start by unplugging the trailer harness
and plugging it back together without the trailer wiring and see what happens. Really straight forward once you see it.

If you complete what SDspeed says, and the problem goes away, it is often an short between wires in the harness. Most problems with tail / brake / turn signals in aftermarket installs is due to corrosion etc due to poor workmanship.

I'd replace the harness, and make sure all your connections are well covered. IF you live in a snow / salt environment, I'd go double on the coverings, ie tape plus liquid tape, etc.

RTFM is right! If there is any indication of wear or corrosion just get a new plug-in harness from a site like eTrailer and be done with it. There not expensive and easy to install.