.....and I have a sub enclosure in my square...behind the seat that has a pair of 12" subs. I built it myself. I had no luck finding a behind the seat enclosure that would accomodate my subs. I took basic measurements of the length, height , width at the top and width of the bottom with the seat adjusted and tilted back towards the window. I went from there with my box build. If you build your own enclosure use mdf board, not plywood! Mdf seals better and although heavier will carry the sound much, much better. Seal all of the seams and joints with silicone or latex caulking. You may want to add a pair of "blow by" holes at the top corner above each sub in the box.. These holes allow ventilation and will als help the sub breathe during operation. If the subs sound better keep the holes, if not plug them. Depending on your amperage (I have 1000watts..) running to my subs, it will hit pretty hard with the enclosure. when my box was complete, I picked up some speaker box fabric and fabric spray adhesive to wrap the box as well. Currently with the setup I have, the back window vibrates fairly violently with the bass along with everythine else at mid volume.
If you had more space, like in a k5
blazer. ..
Ground shaker pbp212.............