If you look a the page http://www.volvospeed.com/Repair/sc816.php, you will see a later radio that has a similar release system to at least to my '95 radio - the little U-shaped arm that is shown at the bottom right of the front is used to pull it out.

It may be that the antenna connector is bad. I would try pulling the radio out (don't disconnect the power unless you have the radio code) and flex the antenna connector while playing the radio to try to reproduce the problem. FWIW, I see in the records for my car that an *AM* problem was fixed through tightening the connection at the antenna itself.

BTW, solder joints can go "open" with age through fatique. Unless someone poured some metal shavings in you souldn't have a short. This doesn't decrease the debug pain that someone will have to go through if it indeed is on the circuit board. Good luck.