To my knowlegdge, there are only two MP3 players designed for car use that run under Linux...Cajun and Route 66. The Cajun site has been down for two weeks, and I'm not holding my breath. Route 66 needs some refinement...a new version is coming out shortly. Are there any other choices?
System Comp V3 - In progress.
Low power MB with C7 CPU, DC-DC PSU, car ECU link, USB TV, GPS, 7" TFT, Wireless, Voice.
There's also IRMP3. http://www.dpotter.com/irmp3/index.html
I use IRMP3 and I think it's pretty good.
It it's a frontend for mpg123 using LCDproc to display on an alphanumeric lcd display and lirc to recieve commands from an infrared remote controller. (lirc supports many ir receivers, including irman and simple homemade ones). In addition, IRMP3 has a modular architecture that makes it easy to add your own features. I recently wrote a search module that uses the numbers on the IR remote like a telephone keypad.
If you're going for a TFT LCD, you might want to check out <a href="http://www.ryanspc.com/">Ryan's CARAMP</a>. I like it alot, but there is a small problem of it crashing on some systems (mine included, but it's the best looking SVGAlib player, so I don't mind too much).
200mhz, 20gb hotswapable drive, 128mb, Winamp w/full-screen vizes, (upgrading to WinAmp3 in near future), 5" LCD screen, controled by numberpad, Sproggy PS under construction, custom mounting case under construction.