Hi Heiko,
I found this forum via Google. I can't locate my owner's manual because my Boxster was in the shop for a full year! Long story but needless to say it needed a new battery and the radio is asking for the code. Following the instructions at the beginning of this post - this is my information:
Becker CR-220
Model 4462
Serial 25005888
To note on the serial - it *looks* like the second digit is a 5 and the fourth is a 0 (zero). The LCD on the radio is kind of not working right (e.g. some LCD elements are no longer lit) but it was working fine audio-wise before going to the shop.
Any help is sincerely appreciated!