I have a 2004 Chevy Tahoe, which apparently has a wiring harness problem.
I have two trailers, both with new wiring harnesses and light assemblies.

ALL Tahoe lights work fine without a trailer connected.
Trailers have good grounds.

The Tahoe plug at the back tests “good” on all connections UNTIL the trailer is plugged in. Then the lights go crazy.
The Tahoe ground then tests “hot”
Tahoe lights: none on trailer
Brakes lights: none on trailer
Left Directional: good on trailer and Tahoe
Right Directional: none on trailer; Tahoe works, but both tail lights come on
Brake & left directional: Trailer has left directional; no brakes; Tahoe ok
Brake & right directional: Trailer has LEFT directional; no brakes; Tahoe ok
Lights & left directional: Trailer has left directional; no tail lights; Tahoe ok
Lights & right directional: Trailer has no lights; Tahoe has tail lights, but right directional stays on and does not blink

Both trailers have the same performance.
The Tahoe wiring has never been damaged or under water, and has worked fine for five years.
Can it be the right tail light module on the Tahoe? Wiring harness?