It's something electronic, I think it may be for an antenna because the wires go up to what looks like where you would screw on a radio antenna. It's all on the drivers side and the antenna thingy is on the rear side window. Does anybody else have these or is it aftermarket?

Factory amp maybe? I don't think it would an antenna but I don't know for sure

Factory amp

thats where the factory amp went. but amps dont have antennas. probably the remmants of an old car phone or cb/ 2 way setup.

Pic? I've never seen a black box under the seat of a XJ before so I'm kinda curious.

Sorry no pics I'm not very tech savvy, but it does go to the antenna mount thingy. The mount for the antenna? Is on the front top corner of the rear side window. I think it's for a cb or some sort of radio like ktm said, But any input is welcome.

Seems to me they put some part of the abs system under there, I don't know if your's has abs though, mine dose'nt so I can't tell you for sure.

With this being the case it's definitely the amp/receiver for a car phone

No abs here, but I gotta 8.25

It's there to record what went wrong if you crash, emergency responders can use it after and see. lol

If it's an 88-90 Cherokee w/ABS, it's the brain for the ABS, which is junk. Called "Jeep Bendix 9 ABS", took mine off and replace with power booster and reg brakes, works fine.