I was thinking of having just bass in the rear doors, using 6.5" (16.5 cm) woofers,
Now I'm going to sound all elitist, and everything... I ask forgiveness in advance. :-)
A 6.5" speaker is not a subwoofer, no matter what it says on the box or in the advertising. A subwoofer is a speaker designed to produce strong frequency response in the 20-80Hz range, and a 6.5" speaker isn't going to do it, particularly when it is running free-air inside a door panel.
If you want strong bass (and I am not talking about the "boom-boom-boom" bass you hear coming from that low-rider down the street with the reversed wheels and the blacked out windows) you have to pay the piper -- in this case, the piper wants about one and a half cubic feet of your trunk space in order to place a 10" or 12" subwoofer.
Unfortunately there are no shortcuts to good bass response but with some thought and creativity you can minimize the impact the installation will have on the vehicle itself. Talk to the pro installers -- they have thought about and solved installation problems you didn't even know existed.
tanstaafl.
"There Ain't No Such Thing As A Free Lunch"
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"There Ain't No Such Thing As A Free Lunch"