try: put the key in the ignition and turn it, switch on the radio and at the same time hold in the [AS] (i think) until it askes you to enter the code. use the numbers to enter the code or the seek/volume button. then [AS] again and you should be on your way.
This is what i did to my vauxhall tape player so it should be sort of the same process. The button you may need to hold in might not be the [AS] but a similar one. See how you get on.
