You have to remember that the sound will be muffled by the seat itself unless they are near the outer edge and then you should be able to bounce the sound off the sheetmetal of the cab.

In my standard cab 2000 toyota a few years back I ground down the back of the cab smooth, layered it with tin foil taped in small pieces and fiberglassed a few layers across the back. So I had a perfect smooth fitting panel which I then attached a wooden box to that held 2 6x9"s, 2 4x6" plates with tweeters and a 12" subwoofer and then carpeted and screwed it into the cab with cotton batting as insulation between the cab and box. Way overkill but it made for great sound when having bbqs or camping!

Professionals will actually use the cars design to bounce the sound for better quality. Ive seen this used when placing tweeters in the dash or doors where they bounce it off the windshield.

Hope that gives you a little help!

Jeff

My 1953 Chevrolet
1947.1 Gallery
1972 C-10 1/2 Ton & 1972 C-30 1 Ton