A box is not out of the question. There a alot of members here that have subs behind the seat of a regular cab. It's doable. I would check into shallow mount subs. JL Audio, Kicker, Rockford Fosgate, Pioneer, & Clarion all make shallow mount subs. You could probably run a couple 8's behind the seat, also. If you do some searching on the site you can find some pics of boxes behind the seat.