I need some advice on High End Car Audio...

BEWARE, although the rewards are there, audio nervosa is an even greater threat than with 2 channel home stereo. With EQ & crossovers a given, a zillion speakers including multiple subwoofers plus their associated powering amps...well you see what I mean! Having said that, a trip to the bookstore will get you started as there are some decent mags. devoted to the "hobby". As you can probably infer I'm a devotee, Good luck, and by the way if you just want a reasonable upgrade for the Bimmer, Bavarian Soundwerks; http://www.bavariansoundwerks.com/ offers drop-in replacements.

Audio Design Systems 255 10th Avenue New York NY, 212-255-8900

These guy did the radar system in my car and did a great job. There a little pricey, but they do a pro. job.

Find a high end shop in your area that carries the Focal line, Focal makes some really nice component sets for automotive use.

You will need to find a great dealer, who has a real understanding of high-end audio. Cars can sound really, really good, but it is very rare to find a car audio dealer that has a clue whatsoever. Most car audio shop owners have never heard a real high-end stereo, and are in the business of making cars boom and bump.

My best advice is that the installation quality is by far the the most important aspect of any car stereo, so make sure you find someone who knows how to make speaker cabinets out of car doors. My best recommendation, is to not commit to a dealer before hearing one of their car installations, and confirming the "good sound" to them, is anything like "good sound" to you.

I went through 3 car dealers, all of whom promised the world, and delivered extremely sub par, bordering on painful installations. I finally found someone on a car audio forum who really knew what he was doing, and was able to build a car audio system that even an audiophile can enjoy.