Great stuff guys.
Remember - if your headunit can do this - to add a high-pass filter to the regular speakers, so that they dont try to play the same frequencies as the subwoofer. This will give a much much much
cleaner sound, and more headroom (=louder).
I used to work for Pioneer so I had may share of big subwoofers, and cars with completely rebuild limegreen fibreglass interiors to make space for as many amps, subs and 7" screens as you can dream of.
when I got my GTV I wanted good sound in my car, but I prefer to keep things a little low-key.
...So my solution was to use the rearspakers as subwoofers. (rearseats arent really used, since there are no hobbits here)
I replaced the doorspeakers with a good 13cm component set (an old pair of Pioneer TS-C series), and for the rearspeakers I got hold of a 17cm component set form Pioneers TS-A series. This has strong focus on the bass, but the tweeters are not much of a thrill, but I didn't need them anyway- woofers mounted in the car, tweeters in the bin.
So the trick is to get a headunit which can: A) play the rearspeakers as subwoofers B) separate the subwoofer-frequencies from the front speakers.
Pioneer DEH-8400 can do this, but probably many other headunit of various brands can do this, and of course many other speakers are capable of acting as subwooferes ( JBL GTO series, Focal, Morel....you name it)
Shoot if you have any questions. I know a thing or two about stereo