From what I've heard, the best sound is achieved through directing the low frequencies into a solid part of the car. My old 2x10" setup was loudest when the drivers fired as close as possible to the rear of the car, almost touching the back of the trunk. Moving them so the box was touching the back seat just made them rattle. Now I have a single 12" firing down into the floor of the trunk. Still sounds good but I'll bet it could be louder.
