most of the software are serial no calculators, on the head unit there is a long serial number and all you need is the last four numbers and press calculate,(ie clarion) but it depends on what make and model you have obviously, some might ask for the bar code, usually the bottom bar code number, and some makes you will have to open the unit up and find the pin connections or have to buy expensive tools to be able to dump the file and replace them with new ones, but most of the software you get from fleabay etc are for older cars and and some dont even apply to land rovers etc . you can even find the exact software that you buy on fleabay on many torrent download sites for free. believe me i hate paying for the code but in the end sometimes it is quicker.