4 years ago I had a computer in my car. Ran Windows 98 with winamp.exe as the shell. I used a plugin for Winamp that let me control the song selection, volume, etc. directly from the keypad without a monitor. I just kept a printed list of the songs on the hard drive, and used the keypad to go straight to the right song.
I googled a bit but didn't find any new development...all of the download links for it seem to be dead.
I'm sure somebody has already done this, though...you might check out the forums at
http://www.mp3car.com