Question about Car speaker design.

I spend 4 hours a day in my car minimum. I don't buy new, since I wear the things out at the 45-60,000 mile per year rate. I find low mile older cars cheap and quickly rack up the miles. The latest "cream puff" for me is a 94 Cutlass Ciera, right from Grandma.

I have a nice Alpine in the dash with pretty green lights. A set of 3.5" Pyles and sep. tweeters bi-amped with a rather pricey crossover have made the front very listenable.
The 6X9's in the rear are not acceptable. Even after trying many different options. I find they provide "boom"...but to call what comes from them "bass" would be wrong.

I have 5 different sets of drivers lying around. The 4" Fostex I have been playing with and the 3,4 and 5" Stillwaters.
Since the Auto market seems to be dominated by the "neon-glow-blow-your-body-gaskets-SPL "competition" mentality, I have not found anything in two weeks of searching that can fit the bill.

There are a ton of sites with info on the 4" fostex for example. But none of these designs will fit in a car.
I am looking for something that I can fit into the rear deck of the Olds. Even PVC designs would be OK, cause I don't mind hacking the rear deck up...but it needs to fit in the less than 24" tall space that GM has allowed me to work with.

Anyone built thier own system for the rear deck of a car?

Any enclosure ideas?

Helpful sites?