I found the only way to get a "decent" sound in my last car (couldn't be bothered spending the time or money in this one!) Was to do all the following:
1. sound deaden pretty much every panel of the car - full interior out job and took a week to do, this helps the midrange and vocals no end from background road noise and stops panels resonating from bass and sounding horrible.
2. create new driver mounting points, and farbicate mounts/pods from MDF) as the stock locations very rarely offer decent SQ.
3. run active - I ran a 3 way active front end (DLS components, DLS A series amps and Alpine HU with adjustable crossovers, slopes, cut off's and time alingnment) and a 12" sealed sub in quite large box in the boot.
4. Use scopes and meters to find dips or peaks in frequencies throughout the car and eq these to be as "flat" as possible for a start point/base point - you'd be suprised how small range of frequencies can peak in small cars.
The sound was very good (for a car!).
Don't forget that most car speakers are designed for "open baffle" or large enclosure sizes due to not being completely sealed in most cars, so using non-car speakers designed for small boxes etc might not sound great or give the full frequency response you were after. Of course if you select your own drivers with this in mind this may not be the case
