Interesting. Believe it or not, we have done a lot of thinking about car stereo. No plans yet.
However, here are some thoughts:
The best possible car system would be a computer. You could store your entire CD collection on a hard drive.
If I were to do a Car stereo system, I would use the 3 inch drivers we will be using in our Multi-media system.
These only need about .75 liters so they could be housed easily. I would then put 3 inch in adapters for various cutouts for cars.
The 3 inch TB speakers sound better than any of the system I have heard so far.
I would then use the same subwoofer we use with the Multimedia subwoofer. It would go deeper in a car because of the cabin effect you get in a car. We could get down to about 30 Hz -3dB.
I would want to have a system that was DVD or movie ready. Using a computer, you could have a system that provided SRS. With SRS, you could decode any audio source from 2 to 7 channels.
The most important thing is that it shouldn't be expensive.