While waiting for your reply, I figured I'd just throw out some things that will help you.
First thing to do is purchase a very large amount of sounddeadener and take this car down to the shell (on the inside), and lay some serious effort into deadening the car. This should be done with ANY serious effort to improving audio performance in a car. Good 'ol Dynamat is fine, but there are better contenders out there for the money. Expect to spend around 100-150 minimum on this part. Brands to look at: Raamat (best deal in the house) and Second Skin (best performance, light weight.) What you want to do is place this stuff on the inside skin of the doors (by opening up the door outer skin, and unbolting the plastic piece in the middle), then cover just about every surface you can with the deadener. It should be about 10 square feet or so per door. After that, you want to use Raamaudio's Ensolite product, combined with their spray adhesive, and cover all your work with Ensolite. This combo alone is incredibly effective in making a car quiet, and much better sounding (since the lack of sound quality is often attributed to resonance and distortion created by unwanted vibes.)
After this, you should evaluate the system again, and determine if you need to go to the "next step", which is to overhaul the system with new speakers, etc. But first, do the above stuff, and report back. I'm sure you'll be impressed, but I'm unsure if you'll think it is enough. If it isn't, then you'll at least be done with the hardest part of doing a good system install, and the next parts will seem like cake.
