The SERIAL NUMBER begins with M ****** and is found either with the car`s paperwork(usually in the Audio Guide) or on the radio casing.(which will mean that the radio will have to be extracted from the dashboard using a pair of Universal Extractor Keys ...available on Ebay or from Halfords etc)
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix in-car audio.