Depending on the make of radio , what you may have to do is to switch it on as though you are going to enter the code ( pressing ON switch +another button perhaps? see below ) then leave it for at least one hour . IMPORTANT ..Do not disturb it as it will start the one hour again )
Entry info here
http://www.davidsonelectronics.co.uk/code_entry.htm
RegWorts, June 2008