Hmmmm do you think you have an 8 track cartridge stuck in there causing it to lock up?
Sorry I just couldn't resist

I think as with any sort of discontinued technology you're on your own to tear it open to figure out what is the culprit. Have you tried searching
http://www.car-part.com to see if you can find a cheap replacement? All you need to do is find one that is mechanically sound, then replace your front face and/or missing/broken buttons as needed with your good parts.
Good luck with it. I remember our old 1987 4Runner had a doublebay radio + cassette player that worked flawlessly till we sold it in 2000. Have you peaked inside it with a flashlight? Perhaps it's something simple and easy to extract/fix with needle nose pliers?
Good luck man!
BigMike