I had been messing with my other easier to move box before it blew out, and i found that if you face it toward your trunk it is a little bit louder, not that much though, but i would seal it off and face it in for your best results imo.
Every car is different. I found that when i ran to 12''s in mine,box put behind rear seats,with subs an port facing rear was the loudest. Now i run a single Q15'',sub up,port facing cabin is the loudest. So i think its just trial an error realy. Do your rear seats fold down?
As sean said every car is different, the only way to know is to experiment.
But with rear seats that dont fold, something that will help is making a passage between cabin and trunk. I usually do this in trunk car installs: Take out rear speakers since rear speakers do nothing but screw up soundstage, put the grilles back with no speaker there, boom, stock look and two decent size ports into the trunk.