By saying that the tape player works, do you mean you get sound out of it. If so, have you looked at the connection on the antenna itself. Often times the electrical connection is undone because there was a short in it that ran the battery done when the car was off.

If there is electricity at that connection then you need am ohm meter to ceck that there is signal coming from the antenna and if there is then check and see ifthere is signal at the back of the radio. Check the fuse at the back of the radio and on the amp in the trunk. I think on mine that rear amp has a plug in to make the antenna work so check that it is plugged in. Good luck. It wont be hard to find the problem but it just takes time. I would start at the amp in the trunk....