Is This A Valid Assumption?

[The Wizard]

Another weekend wasted chasing electrons!

If when hooked up to one vehicle the trailer lights function as they should with no anomalies with the trailer lights or vehicle lights and then when hooked up to another vehicle the vehicle's backup light come on when the other lights come on, can one assume that the trailer's wiring is OK and the problem lies with the vehicle's wiring?

I can not figure out how voltage can be getting to the backup light just because the trailer is hooked up. Can someone DRAW me a picture?

 

[LCOwner]

then when hooked up to another vehicle the vehicle's backup light come on when the other lights come on, can one assume that the trailer's wiring is OK and the problem lies with the vehicle's wiring?

Are you running a light converter on the tow vehicles? Problem may be there. Also make sure you are running a ground circuit between the trailer and tow vehicle and not just relying on the ball to ground.

 

[The Wizard]

There is a ground circuit between the vehicle and trailer.

 

[se2000]

You 'll have to go more into detail to be sure. But you could make that assumption as long as you realize you may have to change your assumption. When things look flakey, poor grounds are usually at fault