Car is a Subaru BRZ. I currently have a pair of Infinity 8's in a ported box, facing forward firing into the back seats. My original plan was to fold the back seat down, and have the subs and port firing directly into the cabin. This sounds good, but with the seat down the seat and/or latching mechanisms rattle and is really annoying. So I end up having the seat up most of the time.
The trunk is not big, and the space between the front of the box and the seat back is kind of wasted. So I had the thought of attaching my amps to the back of the seat (near the trunk floor were the seat back angle provides more room) and having the drivers fire into the trunk. If I were to do this, I would make a new sealed box housing a pair of ID12's. However, the box would have to be as tall and wide as the trunk is, so there would be no "free air" between the back of the seat and the trunk area. I'm not sure if that will cause a problem or not.