sounds like maybe it's the anttena cable, as horatio said. my dad's 86 f-250 had the same problem and i tried everything i could to fix it. i even put in an old deck i had lyin around, but that didn't work either. not sure about the ZJ, but on my dad's truck the problem was that the antenna cable had came loose. i took the 4 screws out that held the antenna to the hood, and there was no cable attached. looked in the hole, and there was the cable with a banana plug on the end. fished it out, plugged the cable into the antenna base, and reattached the assembly and the radio worked fine. as i said, i'm not sure about the ZJ, but it's bound to have a similar arrangement. just check that the antenna is connected at both ends. if they are, i would use an ohmmeter or continuity checker to see if the cable could be shorted or broken. your am radio should work fine, because there is a seperate ferrite antenna inside the radio that is unaffected by whether the aerial is plugged up. hope this helps! jairus
'81 CJ-7 Got it when I was 16!
'06 Hummer H3, boulder gray, Adventure Package(electric rear locker and 70:1 crawl ratio BONE STOCK)
'71 Kaiser-Jeep M35A2 Deuce and a Half cargo truck, with winch. <for sale>