Hi Alison, you will have to go to somewere like halfords and buy the extraction tool (£5) they go into the four small holes in the front of the radio push them in and then pull the radio out the serial number will be on a sticker and engraved into one side if your lucky you might find the code written on the unit!
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix in-car audio.