Ah, the real details.

All good in car systems have pretty displays with easy to follow controls and a remote. They also fit into standard DIN slots in car consoles and look like that was the plan to begin with. In fact, car stereos do these things very well. So instead of reinventing the in-dash-entertainment it makes sense to use one. This means you get things like a radio and CD player for free, along with a professionally designed bouncing graphic or somesuch.

But how do we tack our own MP3 player into a proprietary self-contained (and sealed) system?
The same way that the companies themselves do - make a CD Changer. Only one that's fancier.

To minimise the circuit building, and the work involved, we'll use a laptop. This approach has a few advantages:

• already relatively compact
• biult-in support for power-saving
• built-in support for running of battery (ideal for shutting down without requiring car-power)
• can run a standard OS which provides access to mass storage such as a harddrive (and flexibility in control)
• storage components (relatively) easily upgradable

So we have the two basic components already made for us - the car headunit, and the mp3 player. All that's left is to connect the two of them.

We picked Sony as the headunit after the success others have had interfacing to their "UniLink Bus" which is used to control their CD and MD changers.
As they intended to use the protocol for MiniDisc (MD) as well as CD changers it already has support for named tracks, 'lots' of tracks per disc and many discs in a single changer.

So the plan is to take a regular laptop, a regular (Sony) headunit, with changer controls, a small amount of 'glue' to link them together, and we end up with a nice in car MP3 player using standard parts, but with the potential to be 'infinitely' upgradable:
Want to view movies? Use a laptop with 'TV' out, add an LCD tv with video-in, select the track on your headunit and the sound will fill the speakers as the movie rolls on your tv.
Too many tracks for your player? Put a larger harddrive in it (or use a laptop with support for more than one HD, or removable discs - 4G of music on a DVD, anyone?)
Fancy new format supercedes MP3? Update the player program and away you go!