I actually prefer to hear the rumble of the car, but...
A car with Concert Sound II will have speakers in the doors.
The rear speakers are 4x10", but that doesn't leave many options. There are adapters for 6x9" speakers.
You could always do something like Richard Clark did.
http://www.usdaudio.com/sw/cars/buick/
http://www.mobilesoundscience.com/f3...1992-ca-e-447/