Looking at these sites has gotten me thinking:
mp3Mobile
MpegBox 2

Of course, those two blokes took the most respected path to geekiness by building their systems from scratch. However, i'm thinking that all that would really be needed is a fairly inexpensive laptop with at least a Pentium 233 MHz processor and a 4GB HDD, a cigarette lighter adapter to run the laptop off 12VDC power and one of those Sony headphone-tape cassette adapters to play audio through the car audio system.

Running Windows 9x you could then alter the SHELL= command to point to a user interface written in VB that automates WinAmp. The hardest part would be finding an input device so that you could skip tracks.

However, a rudimentary working system could be up and running with no interface whatsoever.