MPD would be a nice base for an open source car player. It's client-server based; the server part does a lot of the goodies we like, like gapless playmode and crossfading. The server could almost be used as-is, only a new client would be needed.

The nice thing is that MPD can talk to several clients simultaneously. So you could have one client doing the regular UI, one do the remote control handling, one handling the stalk controller, and others that will handle your custom control methods.

Pim