thanks guys for your responses,
For an update,
My plan was to buy a sub and mono amp, and have all of that installed nicely in the boot, keeping all the speakers etc, toward the front, just leaving the sub stuff in the boot. that was my plan. But if the ol' back of the back (on mdf) seat is the answer, then so be it. But keep me posted on any other ideas you may have.
Cheers!
what about underneath the rear seat? is there any roon?