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You can fit subs behind the rear seat without moving it. I seen boxes w 2 12"s, 4 8"s and 10"s. I have the kicker 12" compact wedge shaped box (Kicker TCVT12-2 Car Audio 12" Truck Shallow Enclosure Box) on the drivers side that sounds great. You only need to cut the ridge off of the carpet on the rear behind the seat and remove the vent. The box goes for about $200 and it was quick and easy. If you want a custom box look on Ebay or check out www.supercrewsound.com But for the money and easy fit it's hard to beat the Kicker box.
I took the rear seats and raised them up three inches and moved them forward an inch. If you look at the rear seats ther are two brackets on the base. The first bracket is contoured to fit the floor while the second is a 1x2 inch bar that is simply box tubing. I took a piece of two by four box tubing, ruberized undercoated it, and wrapped it in automotive carpet from napa. I used that to replace the 1inch piece mounted to the seat frame. Had to buy two extra bolts and drill a couple of holes after I cut the two by four to size. It only took me about an hour to do everything. Pretty simple. While I had the seat out I also used some sound deadener on the back wall and the floor since ot only takes a second to pull up the carpet.
yes i have put 4 10's behind my rear seat.....like mentioned above try e-bay or supercrewsound for some decent boxes, or make your own of 3/4" MDF. also, you will recieve a few more answers from posting in the Sound Forum on the main page.