Sub bass is all about moving air ... no matter what direction the driver is directed ... The downfall to upward firing onto the back seat is the restriction of air travel ...
lift the seat up in stowed position, problem solved ... inaffecient and ineffective for most practical use ... You do not see many of these at all ... unless they are ported through the sides ...
Downward firing is better and will fix this problem, but you have minimal driver choices because of depth ... most of today's sub drivers are too large to fit these enclosures ... Port tuning can help these get stronger, but neither of these enclosure setups will benefit you in a outlaw bass competition ... both will sound good to normal music playing and get fairy loud ... but with heavy bass music / rap, you might be a little dissapointed with these ...
We all know my favorite enclosure style for these vehicles ... the Blow Through style setup ... but ...
One of my favorite enclosures for these body style trucks is the center console enclosure ... tons more room, you can downfire large drivers, forward and rear facing ... just have to trial and error to see what benifits that particular vehicle ... you can fit up to 4 8s, and possibly 2 12s ... and when mounted properly to the cab, and on adequate amp power ... the ones I've see kick total ass !!!
and most i've seen, you couldn't even tell that they were sub enclosures unitl the BASS cranked up !!!
again, just my suggestion here guys ... Best of luck ...