I don't know how much room is under a typical bench (never measured personally), but I have seen 1 10" woofer under one. If this is a route you can go, I would recommend the
RE Audio REX series 8" woofer. I have heard the 10" and for the price (about $50), I don't think it can be beat. The 8" only requires .35 cubic feet of enclosure sealed, so I would imagine you'd be able to get 1 under the seat incredible ease and more than likely 2 with 1 under each side.