Try a peak at Crutchfield.com for ideas. 6X9 bass speakers are only made by a few manufactures. I think you'll find them by kicker and ummmm, crap, brain fart.... ummm, AAAaahhhh, dang.....
Theres also this option of a free air speaker. You could just make some simple bar mounts, like a angle iron cut down to fit the span of the truck renfocement bars behind the rear seat and box it in to fit and screw the speaker to the bars.
I have used the Infinity brand before, I like the Kappa line more so, didn't see it though. Theres other brands listed for free air there.