Is it a Philips radio? You need special tools (fork like prongs) that go into the two holes either side of the radio. You can pick these up at any good car spares shop.
Hi there, if it is the standard Philips/Rover radio, I made my own tool with an old coat hanger...simply cut it up into two 8" lengths, bend it round so that the ends fit into the holes (underneath the plastic caps on the left and right of the radio front), push the "tools" in as far as they'll go and then lever them outwards and pull the radio out (usually by the tape slot!)