2010 E350 Trailer Parking/Tail Light Problem (SOLVED)
 

Hey everyone!
I'm having trouble with my trailer parking/tail lights and I'm hoping you can help me. I know for sure it isn't the trailer, the lights work when connected to my other van and also do not work with my camper. I've replaced every fuse and relay in the PDB I think are associated with these lights, #6 relay, #17 50amp, and #57 15amp, also replaced the entire plug system (7/4 pin combo) and wiring harness, but no dice. Brake lights and turn signals work just fine as well. Is there possibly another hidden relay, which is not listed in the owners manual? Do you have any ideas? Thanks!

So, apparently there is an auxillery battery in this van which I had not previously known about. It's located on the right side (passenger side) of the transmission and this battery connects to a relay which runs the trailer parking/tail lights. Turns out that this particular relay was bad. Everything works fine after having the relay replaced. Sucks that I had to pay a dealership $150 for a diagnostic and another $135 for the relay, but hey, problem solved.