The code should be on the detachable inner first page of the Audio Guide. If not then it will be on the radio casing so the radio will have to be extracted from the dashboard using a pair of extractor keys (Available on Ebay or elsewhere) . You could try using 4 pieces of coathanger wire/small screwdivers/allen keys or even 4 darts but it is a fiddly job without the proper extractor keys. (An assistant is also useful pulling the radio out whilst you are fiddling about with the extractors)
RegWorts, June 2008