[Graham_A_M]

^ thanks, I sure used to be back when I was around 18 or so, but I forgot about 90% of what I used to know, now I'm practically an audio dumbass in comparison.

6x9's are not subs. there have been ..... I think two maybe three companies that have built an ACTUAL 6x9" subwoofer, but they are extremely rare, and I cant even remember a single manufacturer right now, Although I think Earthquake did in the late 90's early 2000's I'm fairly sure.
But 6x9's do NOT have the qualities that make a subwoofer, a subwoofer. They are typically designed for free air use (as in the back panel of a car) and do not have the techincal parameters that make them usuable as subwoofers. I can honestly say to you that you'll be wasting your time if you expect results from regular run-of-the-mill 6x9's placed in boxes, way back behind your rear seats in the back of your 2nd gen. (designing a box that'll make them usuable as subwoofers is damn near impossible as incredibly few manufacterers list the technical data of those said speakers that enable a person to design a box for them that'll enable them to produce decent bass

Just go with some decent 8" 's. honestly. There is that new post about those 6.5"s that Jaejw1 posted, I'd check into those as I didn't even know those existed, but you best best are two 8"s in a corresponding .5 cubic foot box, that would work beautifully in those said spaces that you want to place these in.