Unfortunately the majority of SIEMENS/VDO codes (including your CDR500) will NOT be available for free (or cheap) from anywhere on the Internet as access is required to the original factory database...which is not commercially available..
Try your Main Dealer or a Decoding Service or your local Car Radio / Audio Shop (who are normally the cheapest option) who will connect it to a laptop to read/recode the memory chip (EEPROM)