In my experience, a down firing sub in a open top vehicle like a jeep with out a trunk works far better. I'll see if I can find some pics of the system I had at one point. Couple things I notice, your subs being against the seat like that is not a good thing, you are loosing a lot of parametric volume and the subs are probably not going throught the entire stroke, especially with a ported enclosure. have you had anyone sit in the back seat with those speakers there? Lastly and this is only because I am super anal, your wiring on the back could be cleaned up a bit.
sounds good though!