A lot of people put 10 inch subs in the rear hatches. Scott has some pictures of this. I think you could put a single 12 in there, I tested a 12inch solobaric and it looked like it would fit. The air space under there seems close enough to a cubic foot for a typical 12inch. Just save your old hatch lids for resale time, you can make new ones to cut up for the subs.

Roger