You can spend as much if not more on a true audiophile system for a car. The very first thing that you need to check is whether or not you can remove your existing head unit (the stereo) and retain all of the functions in your car.
Let me give you an example. My Cadillac SLS has the fuel pump and the OnStar system that runs directly through the head unit. That meant I can't remove it for something better.
The next thing you need to find out (assuming your head unit can be removed) is if your head unit is integrated or has the amps separated and tucked someplace else in the vehicle. I've seen amps under the console, in the trunk, behind the glove box and under the seats.
The reason I bring this up is because most of the current head units get bolted in and then the installer plugs in a adaptive wiring harness that still has the existing amps in your circuitry. The vast majority of the existing amps will limit your overall power and will definately effect the final sound (since you are running signal through them). You can have the existing amps bypassed.
You need whomever installs your new system to run new speaker wires to each of your new speakers. You don't want to re-use the factory wiring because it sounds pretty dreadful. Though you can do an active crossover mounted in the trunk, most decent head units now come with a crossover built in. In turn, you will need a head unit with at least a single pair of pre outs rated at somewhere near 6 volts (2 volts won't cut it). This insures ample voltage to drive a (near) 20' pair of interconnects going to your sub amp in the trunck.
The size of your sub (and enclosure) will depend on the size of your trunk. There are some programs out there that will simulate the frequency response in the cab based upon the size of the sub, size of the enclosure, size of the truck (in cu ft) and the size of the cab. Sorry, I don't remember the links. The Deware site is a great place to start. They make some killer sub enclosures for cars. Look at the Wicked One over there.
Everything that holds true for home audio holds true for cars. You need to use good speaker wire, good (shielded) interconnects, power filtration and storage. Finally, you need to pick a quality head unit. Clarion makes nice sounding, affordable head units. Forget about the recorders, hard drives and DVD players in the head units. Go for the best 2 channel (only) unit they offer.
Speakers are obviously crucial. To compensate for lower power head units (usually 40wpc) you need to pay close attention to the efficiency of the speakers. You may have to dig for those specs but they do exist (somewhre). If you can find a decent sounding pair that sits around 90db, go for them. Even though the manufacturers state that the head units are rated at 45x4 when you bench them, they usually come out between 15 and 20wpc rms so caviat emptor.
If you go after speaker separates (separate driver, tweeter and crossover) pay very close attention to the quality of the crossover parts. Make sure they don't use electrolytic capacitors. They should use films or better yet metallized polys. Iron core inductors (coils) will saturate at high volumes and cause distortion. Try to find air cores. Cement resistors suck but unfortunately everybody uses them. Stear clear of those add on super tweeters (ick).
A simpler pair of coax speakers can sound pretty darned good if you swap out the cap on the tweeter (they usually run the driver full range), honest.
Bottom line, to do a midlevel audiophile system (right) in a car you are talking in the neighborhood of $2 - $3k.
I went through all of this not too awful long ago. I schooled myself on all the pieces and parts. I had planned on a Nakamichi passive head unit, the Bulter 4 channel (a true 4x50rms) with the tube drivers, a quad of MB Quartz speakers, a Adire Shiva sub and a decent sub amp. Installed with all new wiring, dynamat, interconnects, etc, I was looking at almost $6k. Then I found out I couldn't remove my head unit.
Guess what, I'm still listening to the shitty assed Bose system. Well, at least now I've got Sirius.
Hope that helps.
