I was just looking at the IBM microdrives when it occured to me that they would be a great way to store mp3 files and transfer them between your PC and car.
There is already a portable mp3 player that supports these drives (can't remember which one exactly) but it requires you to hook it up to your existing deck and add a power supply .....
Just load up each 340Mb disc with your favorite music (around 6 hours worth each) and slot them into the car player just like a standard CD.