Does anyone know about the possibility of custom mounting aftermarket subwoofers in the the wall behind the rear seats where the tiny factory subs are? Is there any room in there? I would like to conserve as much space as possible. Thanks.

 

go with free air subs and cut holes in the firewall you should have no probs. I personally just made a box that hugs the wall (as it is we have no trunk space anyways i stopped pretending)
I may go the free air rout one day myself as it looks cleaner