i find that the stock media player has a simple and uncluttered enough interface for use in a car.The buttons areas are fairly easy to hit.

Other suggestions:

Try the Open media player, has the same inteface as stock plus an "entertainment" view which is basically full screen.

Install queenbeecon widget and have a desktop view with custom widgets on for media control via big buttons. I did something similar on my N800 with personal launcher and dbus commands.