Trailer Electrical Harness Short

[CAS138]

I have a Stowaway storage box on my 2012 AI. The hitch and harness look to be OEM. The harness is branded Pollak.

The lights seem to function properly with one exception: The right tail light remains on very faintly for a few moments when everything else is off. It is clear that it remains on with the interior lighting timer every time a door is opened or closed, doors locked, etc.

My guess is that I have a wiring short tied into the interior lighting.

Any ideas how to track down and resolve this? I am at a loss as to the location to check.

Thanks,
Curt