Voltage Drop @ trailer wiring harness.......

Got a 2017 TRD OR DBSB with the factory trailer towing package. Here's the problem: trailer wiring is sound, (new on one trailer).......Turn signals and brake lights work as they should, but do not have any running lights. Bulbs are good on the trailers.

Took it to the dealership this morning, they've worked on it all day, and have determined there is a voltage drop of +4V to the tail light leg of the wiring harness. But have not determined what could be causing it.

Anybody else had anything similar?

 

Bad connection or damaged wire. 10 minute fix.

 

All connections are good, all wiring is good. Talked to the techs yesterday, and they've narrowed it down to the relay box and/or wiring underneath the dash.