If you are getting an error message that a light is out and no light is actually out, there could be a ground problem or a wiring problem on the trailer.
Make sure that you ground the white wire of the harness to a clean bare metal surface.
Check the trailer and vehicle side connectors for any corrosion and clean it out. Next, check that the grounds on the trailer are attached to clean and corrosion free metal surfaces. A lot of trailer tail lights ground through the mounting studs.... view full answer...