Got a 98 xj, 4.0, automatic, stock electrical system.
The problem is the rear lights are not working correctly. The blinkers will blink the reverse lights, it's even worse when the market/head lights are on. If I'm in reverse the and on the brake, the right side goes dim and the blinker will dimly light up.
With headlights on, no turn signals on, foot on the brake, the brakes lights work, but the reverse and turn signals will light up dimly.

I'm figuring this is a ground issue but my brother is saying could also be a multi function switch grounding issue. Is there a ground at the mfs? Are there chassis grounds for the rear light harnesses?

I've got video of this on my phone, just figure out how to post them from my phone....

 

Did you buy this XJ from a previous owner? It sounds a lot like the wires may have been messed with along the way. Maybe someone trying to repair something. Have you looked at the fuse box or traced the wires from their bulbs to their panel?

 

Sounds like issues I had trying to put LEDs in my jeep... Never figured it out just put incandescent bulbs back in...