My girlfriend said she liked it, wants me to put one under the passenger seat as well. 
I would do too, but I slide the passenger seat about so much I'd end up worrying about damage to the speaker cone, which as it stands is still an issue for the driver side one when I'm shifting my P.A. around. I'll try and get some pictures later, but about 1" of the top of the speaker pokes up from the back of the driver seat, so I'm apprehensive about letting anyone sit in that side of the back now. Should look into getting some thin wood to make some sort of enclosure. It's just so lacking in space under there.
The method I used to get it under there:-
Step 1) Bend the seat slide wire so it doesn't catch on the speaker.
Step 2) Remove the carpet wadding or what ever it is around the speaker area.
Step 3) Tie a big knot in a piece of string then thread it up through the carpet.
Step 4) Tie string to the speaker tightly.
Step 5) Profit.
Simple bodge, and it works.
