Behind the seat or port through the rear deck should result I less rattle, less need for mastic.
Also when you make sure the sound is getting in the car you can get more impact and greater subsonic detail at the driver seat
Both set-ups should result in more sound in the cabin and will not be as loud outside the car as firing a sub to the rear.
I'm told those rear vents allow you to slam your trunk. If you simply close the vents air will trap inside creating pressure preventing you from slamming your trunk close. With the vents closed the harder you slam the harder the lid should spring back at you and not close. I cracked half of one open and closed the others. I also don't slam my trunk, but I can.
