You need a long (14 in?), thin (1/8th) flat screwdriver. The screw is right at the back of the radio so the screwdriver must be at least as long as the radio is deep so it can reach right to the back. There is a hole in the middle, bottom of the front face of the radio which may have a plug in it where the screw driver goes in, but these radios are so old that the plug is probably long gone.

jim is right s small diameter flathead with a long shank is required.  The narco part # for the screwdriver is 47189-0000.