I'm 6 ft but my wife is short, so I put a custom box in the passenger foot well. Not sure what kind of space you have behind the seats, but JL Audio (very well respected in mobile and home applications) makes "pancake" style subs.
10TW3-D4 - Car Audio - Subwoofer Drivers - TW3 - JL Audio
I didn't go through all the specs, but see if a design like this wll work for you. I'd imagine other companies make such designs now, too. The other option would be a stout 6" driver behind each seat. Subs frequencies are low enough that they aren't as directional, but you'd likely want two sixes in a balanced setup vs. just one.