To the OP, you can not fit 10" or 12" subs in the wall, nor would you want to, because free air subs sound like garbage and there is NO way to boild any sort of enclosure in there. Your best bet is to get shallow mount subs and do a false floor or something similar for the trunk and hide them there and still be able to use the trunk.

I have heard a cervertible with all new speakers, amped up, and a single 10 or 12 in the rear, sounded AMAZING. You might not get all the bass, but it will be much better...