Hey... this is kind of blue sky but think about this...
You could try running fairly high Qts poly cone drivers mounted on the floor of the truck firing down with the outside world providing the infinite baffle. You would lose a few dB when you roll down the windows.
You'd probably want to shield the cones a bit from road hash but it wouldn't take too much. You could even hang the baskets under the truck if room were a problem and the truck lifted high enough but then you might get trash in the spiders and such if they aren't shielded sufficiently. I think it would be a pretty slick infinite baffle solution.
I'm actually serious on this one...
Any response?
It does work. I had a similar setup in one of my show cars where the mids were mounted in the floor up front with a set of USD waveguides for the highs it sounded really good. Most people dont want to cut up thier car like that though. Too bad because it was one of the best sounding cars I have ever built.