go to www.mp3projects.com , they have a wealth of info about mp3 computer setups. mostly not for car installs , but the idea is the same. also there was a howto that described how to make a boot disk that had a web server and an mp3 player on a floopy. you controled the mp3 computer via a web browser , and you could hack that up to fit your need. but i cant remember where it was.
anyway think busybox and a boot disk and mpg123