I'm trying to diagnose a problem via the telephone as I drive a truck and my wifes 2000 miles away. She ordered and installed a trailer wiring kit that plugged into the right rear (passenger)tailight assembly and now notices she has no headlights on the right side(passenger) is this a coincidence or a problem caused by the trailer wiring?? If I recall correctly it is anftermarket wiring kit from British Atlantic Parts.

 

I guess I should have mentioned it's a 2003 Discovery, stress

 

I highly doubt the trailer wire kit has anything to do with it.
Have her unplug the kit and see what happens, I have one of these kits going on 5yrs and no issues.
It could be a bad trailer harness unit.

 

Right side head light is fuse 22 and fuse 10 on interior fuse box. Earth ground for head light is inside engine compartment, near under hood fuse box, several "black" wires on lugs bolted to metal part of car.