To get an official Birth Certificate for your convertible, write a letter including 50 €uro (or 55 US $) to
Stiftung AutoMuseum Volkswagen
Please do not forget to mention the VIN (vehicle identification number / chassis code) of your VW and your name. Within a few weeks, you will a receive a nice document stating some details about your convertible. To check if it's worth the money for you, have a look at my convertible's birth certificate. Note that I have nothing to do with this service !