This is to the suggestion of putting 8's in the rear door - that is a very novel and imaginative approach to the problem of (1) finding adequate space and (2) concealing the presence of your souped-up sound system. I applaud your ingenuity - but I can think of a few logistics problems.
First, the easier to address problem might be that the door panel is probably not deep enough to accomodate an 8" cone/magnet assembly. This could be abated by building out from the door, in something akin to a kick-out or kick-panel. I don't know how this would play with the aesthetics or the ergonomics of the interior of your truck, plus it would broadcast to passers-by that you had an aftermarket system, which would undercut purpose (2) ---
Problem 2 - Rattle. You're effectively mounting a bass speaker into a metal box. You would have to dynamat the #$&! out of the doors to keep it from sounding ghetto and tinny... plus, I am unsure that the door enclosure would offer enough cubic footage to acommodate the requirements of the speaker.
Problem 3- - Pressure. You've been in cars with good bass. They can move your hair. If your sub was half-a$$ effective, the backpressure could shatter your window in your rear door. No idea how to address this, other than removing the window assembly and blacking in your window space with metal (the van effect) or mounting a stronger pane of non-movable glass in place of your stock window.
All this being said, if you could accomplish this application and address these (and other) problems - venting back-pressure, wiring - you would have a very tight custom design. It would definitely turn some heads - but don't look to this design as a less expensive alternative to mounting a standard enclosure in your truck... doing it right will set you back some jack, guaranteed.
