check out the new shallow mount subs. Memphis has one that is under 3" thick, and they have plans on their website to build a box that is only 3.5" thick or so. Might even fit under your seat?
You can build really thin wedge boxes now that will fit behind the rear seat, it is just going to cost you a little more $$ for the shallow mount subs.
www.woofersetc.com and look under subs / shallow mount. It'll give you an idea what you are in for.
I've got the pioneer 8" shallow mounts in the doors of my wife's miata running 100watts per side and cut off at 250Hz - the 6.5's mid-door and tweeters take care of the rest. They don't hit "hard" like for rap, but they sound really tight for rock, etc.