At the side ends of the radio prise off the plastic bits and underneath you should see metal retainers which are slid up and then the unit can be pulled out. Make sure you have the 5 digit code before disconnecting in case you need to use it again.
Or to put it another way, take a stiff wire coat hanger, cut it into a U shape, and push the two ends into the pair of small round holes on either side of the radio, and it will release and slide right out. Good luck!
If it is the stock unit pictured below with radio and CD player below it, then the procedure is almost as Paul describes, which is correct for the CD player but for the radio you need to take the entire outer trim off.
Sorry , I should have said radio/ cassette player. It's the standard unit ('97) , the kind with two holes at either side of the top edge of the unit. I think the coat hanger route will be the right one.