Markus Dopp showed me a possible and smart way.
PhatNoise, Inc. from Los Angeles, USA has a solution called PhatBox® which seems to be very professionel and includes itself in the standard audio system of the car.
Updates of the files will be made using a DMS Cartridge and a craddle attached to your PC - don´t worry, you will not have to bring your PC to your car... 
Extract from the website:
"Put a PhatBox® Digital Media Player in your trunk and have all your music in your car all the time, keep your existing car stereo system, and find the right music using PhatNoise Voice Indexing technology."
They have no german dealers listed, but you probably will be able to find some at ebay.
Any comments from experienced Users appriciated!