You just tossing this thing on the back seats or what?
Full size xcabs are a bish to get loud, dude. Doesn't mean you shouldn't try though. Lol.
I'd start with a regular ol' ported SPL design. I'd probably do subs forward and external ports up. ~ 2 cubes gross with (4) 4" or (2) 6" areos maybe 12-14" long but you'll have to play with the lengths to find the sweet spot if you have a meter or can get some time on one.
Subs up and port(s) to the side would be the next orientaton I'd try. After that, if it isn't working, I'd try to force the wave to develop in a 4th order. Big ratio, bog port. Probably .25 sealed each, 1 cube ported. 50hz or so on the tune. That should put you right around where full sizes like to peak.