Actually it's a ZJ, but that's irrelevant. I suspect the dimmer switch is shorted. Another thing it could be is the tail light harness is shorted in the back where the trailer module would connect if you have one. My XJ has a rotted fender liner on the driver's side rear and water got into the electrical harness back there and shorted out my trailer lighting module, toasting the harness connector. Here's a picture;

 

 

The trailer module is behind the cargo area side panel just inside the hatch. In my XJ, it's on the driver's side. It may be on the passenger's side in your ZJ, IDK. The tail light harness unplugs from the body harness in that area and the trailer module plugs into both ends of that connector. If your body harness is set up for a tow package, that body harness plug will have about 8 wires in it, only 4 are used to light the tail light on that side, the others bring the opposite tail light wires to the module IF it's plugged in as well as power for the module itself. One way to tell where it is is right behind the spare tire mount, there is a small grill opening in the panel that is an air vent for the module's heat. In my XJ, there are 4 different harness connector in that area. The one that you want is closest to the tail light. I think the other connectors are for the rear wiper and heated glass in the hatch, as well as the high stoplight and hatch lock. Dumajones has great pictures of the connectors and surrounding area, but IDK how to transfer his pictures to this thread. Check out the XJ Tech thread and look for my thread "I found my electrical problem" currently on page 3.