There are two obstacles to getting great audio in a car environment. One is creating a well designed sound system that puts music into your ears. The other is overcoming the inherent noise floor of a sports car. Limited space hinders the first. More importantly a sports car is pretty damn loud at highway speed, in some cases exceeding 80db. If you develop a system that produces clean audio up to 110db (rock concert levels) your effective dynamic range is only 30db. That doesn't rate HiFi in my book.
My solution won't appeal to everyone but works great for me:
ipod with lossless music library and these:
Westone UMPro 20 - Clear | Sweetwater.com
Full range, flat response, reference monitor quality IEMs that attenuate the noise floor by 25db when properly fitted. This brings the background noise level down to a typical living room so I can listen at normal levels while still getting good dynamic range, and eliminates all the standing waves and resonant frequencies found when measuring the sound space in a Boxster interior. Definitely not a fun car project but if reference quality sound is your goal, this is a good way to go. Just another option to consider.