I have this Infinity BassLink mounted in the back of my '01 XJ and it sounds great and doesn't take up too much space for me. Lots of stuff go in the back of my rig, baseball bags, outdoor gear, hunting gear, 2 big labs, etc... and this tucks in nicely.
http://www.crutchfield.com/p_108BASSLIN/Infinity-BassLink.html
http://www.crutchfield.com/p_108BASSLIN/Infinity-BassLink.html