how badly do u need a sub or 2?
i too have a a ute and i too thort long and hard about a sub. in the end i think that a sub isnt neccessary in that small of a cab.
wot i did was got a 2 channel amp, alpine type X splits and run about 200wrms through each speaker. if you put hte tweaters up near the windscreen u will get a more even sound than if you have 2 subs in ur ear draining out your speakers donw in the door panel.
but if you really like to feel ur music, then a sub must be the go!
i thort about this, and i too came to the conclusion of putting them in the back wall, but u then have to fabricate a box, get an engineers cert coz u cut out metal from the chassis.
an alternative is to put the sub at the passengers feet. in the footwell. or, bcoz these things dont have entre consoles, u mite b able to squeeze one on there.
but i reckon that if u amplify a good set of splits, u will come out with better sound quality and less hassles