Is this a sedan or a wagon you are building the cabinet for ? Facing the ports towards the back of the seats will hinder performance, since it will make port tuning unpredictable (assuming you are mathematically calculating your box/port). Is the stereo system built for show or go ? I would consider an infinite baffle design for a sedan, placing the subs in the rear deck (theoretically) and only a ported or sealed enclosure for a wagon (no rear deck to acoustically isolate the woofer's front and backside).