FYI: My un-tested, un-investigated, theoretical plans for subs in my ute.... :-)
Y'know in the back (supercab area) how there's a parcel shelf (ie. a hollow box section either side of the transmission tunnel to form a shelf). Well, under the carpet, I'm guessing this just consists of a hollow box either side of the transmission tunnel,. which could actually be re-built out of 16 or 19 mm MDF, forming two seperate sub-enclosures. Just shape the outside to the same as original so it still fits perfectly under the carpet. You could put a 6.5 sub in each side - ok, I know it's only 6.5, but you've got two of 'em and I've heard good reports of performance about the JL Audio 6.5" sub. Performs like a bigger sub apparently - reckon it'd have to be good enough for what I want anyway.
I estimated the volume you would achieve with these boxes and with a few interior baffles/struts etc. it'd be fine as a sealed box for those subs.
This would result in a totally stealth installation (eg. with subs mounted in the top and the outside end of each box - firing up - and with colour-coded grills - you'd never see 'em from outside or the passenger seat.) Plus, you wouldn't lose all your cargo space behind the seats - you'd only have to be careful not to put any objects directly on the grills.
Just a though (and future plan for me ;-)