Sorry your here with problems.
Does the back up camera work in full time mode ?
Have you checked for a back up light relay ?
Are you on a Freightliner chassis ?
On my F/L there is a box with relays and fuses in the engine bay, VIM-V , it's about 8" square & 4" deep, with a hinged lid ,4 multi wire connectors, 2 in the bottom 2 in the side. I know the neutral sense & other relays in there, and several fuses .
Freightliner has a toll free , Free , Help line , if your on a F/L chassis they will help you.
1-800-385-4357 have your serial number handy.