IMO if your looking to keep the space in the cargo area and still have the option of easily removing the sub to fold down the seats your best bet would be a self powered bass tube, bazooka is generally the standard in tubes, a 12" or even a 10" would be plenty in an xterra and its extremely easy to remove, they are held in by straps that you mount to the floor or side with a simple wiring harness/rcas rather than having to hide an amp and having all the wires running everywhere... just my .02