I had no antenna in my 90 when I bought it. After getting sick of listening to "Dancing Queen" and the like as this was the only station that would come in I spent the $9 on a stationaty aftermarket antenna. After removing the factory antenna, I noticed that the connectors were a bit different. The aftermarket antenna had the same male connector you would need to plug into an aftermarket radio. But the vehicle cable has a slightly different female end. Nevertheless, I plugged it in and taped up the connection. Now, my FM reception is less than I would expect and less than my other vehicles.
Maybe you have an aftermarket power antenna?