I have a 97 XJ and have been dealing with this light issue as long as I can remember. I would like to fix it if at all possible. Problem that I am having is with my drivers side tail light. If my runners, or headlamps are NOT on and I push the brake pedal, then only the passenger side brake lamp lights up. Nothing from the drivers side. If I have any of my headlamps on and push on the brake then both sides work. Makes absolutely no sense to me. I have looked around a bit, but only found forums on the opposite issue (headlamps on brake lights don't work). Any help is much appreciated.

 

Have you tried reading it with a multimeter?

I would first suspect grounding for the brake light may be bad? You could prove that by making a jumper and tieing that between a good ground point to the ground side of the brake light...

It would probably actually be better for in this circuit to use a 12Volt illuminating test lamp. By checking to see if the lights are getting power with a test light probe when you step on the brake will show you if the ground is bad or not.

 

Thanks I will try that after work today and let you know what I find out.