Scosche makes a subwoofer that gets added to the existing storage compartments behind the seats. I havent heard very good things about it though, crutchfield sell it. I opted to take the plastic out and place a box which is Pre-Fab. Heck I only paid $109.00 shipped to my door and there was no fabrication to any hooks, brackets. I just had to remove the plastic storage bins and the plastic on the back of the seats (which lets sound through better) This guy is on ebay
http://myworld.ebay.com/explosiveaudiooutlet. Check him out.