I suppose the radio is not the original? It will probably written on the side or the back. It's a 4 figure number. It is possible to read the code from the encoder chip. It has tracks cut on the circuit board against it's input pins. Make a note of a) chip number, b) tracks cut at which pins. Look up the chip on the internet at will tell you the code. This is how the dodgy shops do it (you could take it there and pay a tenner for the privelege) hopefully it's written on the side.
Dave P, April 2007