Yeah the Comps will work in either box, and they should have specs on both styles of enclosures. A sealed box is a little smoother (not as peaky). The ported box typically is louder at the tuning frequency of the box, but falls off quicker as the frequency rises. A ported box has to be a little more precise on the sizes, whereas the sealed box can be off by as much as 10% or so.
Like SU said, start with the box at the back of the seat facing backwards, and move it around from there. I, too, dont think you will notice much difference. My car has a ski hole and I built a 5th order box, and located the port in between the seats, so that my trunk wouldnt rattle. All the sound is being fed into the cabin. Downfall: the rear armrest has to be down all the time, not so good if there is three people in the back seat.
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