If it's a factory fitted radio- you need removal keys to pull the actual radio/ screen out, then the radio itself has a removal code. When you take the radio out there should be a sticker with the serial number on it; this will begin with a C, P or L prefix normally. If you search on ebay using 'removal keys' as your search term they actually do removal packs for about 3 quid a pop. This is the normal procedure anyway. The packs come with instructions too. Don't forget to get a Facia Adaptor and the right wires to convert your car to an ISO connection- this is the connection that fits all standard radios if you haven't already. I have included a link with more info.
http://ezinearticles.com/?Car-Radio-Installation-Part-4---ISO-Connections---What-They-Are&id=365580
Good luck!
Hayley, November 2007
