Looks like most companies make shallow mount subs now days
Earthquake makes the thinnest ones at a 2 1/16 mount depth.
http://cgi.ebay.com/EARTHQUAKE-10-400W-4OHM-SVC-SHALLOW-MOUNT-SUBWOOFER-/270692768151?pt=Car_Subwoofers_Enclosures&hash=item3f068bd997
Should be able to get one of these behind the seat of the car I assume.