Unless you want "SPL" as high as possible I don't see a downside to open air. What's nice about boxes is that you can design them so that the speaker output has certain characteristics...but really, how many people in this forum know enough about building boxes that they're going to "tune" them for specific locations and purpose? The couple of people we have...we're lucky to have.
So downsides to open air?...lower output. Lower power handling. Biggest problem: the rear deck IS your actual baffle. Therefore, EVERYTHING RATTLES!!! Every car has it's own characteristics. The vibrations eventually resonate to the rest of the vehicle causing the trunk and panels to vibrate as well. Another problem is that if you do infinite baffle then you want to do your best to keep the area from behind the speaker to leak into the listening area. That means, seal up the trunk from the rest of the vehicle; if possible.
Anyways, doing things right always means a laundry list of check marks but the truth is most people do everything wrong. So my opinion? Throw something in based on reasonable ideas and see what happens.
Get some nice 5-1/4's in the front. 8" subs in the back. Low pass the subs high, maybe 125hz or 200hz w/24 decibel slope and play them in stereo...specifically L and R. High pass the front speakers around 160hz or 125hz with 12 decibel slope and you're done. If you can, seal up your doors so only the speakers are open to the cabin. Also, use plastic cutting boards from wally world for your speaker adapters in the front.
