What type of sub were you looking to use? There have been a few people who have tried installing 1 sub behind each seat. Some have said there is enough room and some have said that there isn't enough leg room.
I think that it its possible if you design it properly and use the right sub. The poular subs so far have been the Earthquake SWS shallow mounts and the Pioneer shallow mounts.
I am thinking about trying these shortly:
http://www.edesignaudio.com/product_info.php?cPath=1_21&products_id=106
The ED designs are very economical and have had rave reviews. At $70 each its no big deal if they don't work. The specs indicate a pretty small box and should work anywehre from .3-.45 cubic foot.
Send me an email and I can email you some pics of boxes behind seats that I have accumulated.