Hit enter too soon, anyways...
The space in there is really small, and it's going to be a bear to get in there and get something in and have a tight fit. If I were you, I'd consider going for either an 8, 10, or 12 inch sub and get a fairly small box for it (Best Buy sells Q-logic boxes that are fairly small). Then I'd get an amp, and attach it to the back of the box... I'm going to get a lot of hits for that, but it works, and if you screw the amp down tight, it won't rattle. If you need to put stuff in the back, the sub will disconnect within a minute, so you can just stash the wires under the rear seats and move the sub wherever.
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