In-dash MP3 players are beginning to appear on the market. They cost about the same as a high-end car CD player and snap right into the dash.
The first MP3 products for the car are MP3 CD players. Instead of creating an audio CD with your MP3 files, you can put raw MP3 files on the CD. Instead of putting only a dozen or so songs on your CD, you can fill it with about 100 MP3 files.