I just placed an order for a thin line TW5 Line 12 inch sub from
JL. Since you guys are trying to find a thin sub that will fit under the seat you guys might want to look into building a box like evilution specified but maybe changing the subwoofer size and type. I am going to fabricate a simple square box, 14 X 14 (maybe a little wider that is 3 inches deep. The speaker is a 2.5 inches deep meaning it can fit in a very small area. I am going to measure out the back and the area behind the seats to see if it is doable. After that its box building time (my specialty).
This requires a very small amount of space compared to a normal sub.
Here is a picture of the sub from JL's site:
