Just bought a slick lookin Q-Logic box for 250.00US (locally in AK, so figure in some UPS to the average 48 retail) that fits a single JL Audio 10" under the "rear driver" seat, and plan to build a custom "half box" to fit four eights behind each rear seat, firing rearward. Will need lots of dynamat....
Comin off a Alpine head unit (integrated steering wheel control...he he) to a Audio Control 4xs active crossover, Gonna power the bass with a JL amp (500w x2) crossing the 8 8's for mid bass (130 to 80) and the 10" for <80hz and it'll stand your hair on end I promise. Then I am gonna run a Rockford power 400 (100w x4) for the mids and tweets two of each...
still planning out the details, but IMHO the options in a crew cab are killer....
Oh PS I am a rocker.... not a hip hopper
and I tink you could fit two sealed box 12's behind the rear seats if you know how to build a good box...