@Brad Stolberg This wats under the rear seat in a Holden VZ V6 Crewman
I was about to suggest getting a flat woofer, like JL TW5, Polk MM series, or similar, and make a flat box to go under one of the front seats, if there's space for it. Also, I like Brad's 8" suggestion, and am planning on doing something like that myself, with a pair of SEAS Lotus RW220 8.5" sub-ish woofers.
EDIT: Just spoke with a friend, who knows that car better than me, and he says there's not much room behind the rear seat backs, so a flat sub under one of the seats or Brad's 8" suggestion would be the only two options confirmed to be viable here.
From what I can see, the rear seat cushion is one piece, and not three. However, if I'm wrong, there's a third option: Convert the car from a five-seater to a four-seater, and build a centre arm rest for the rear seat with built-in subwoofer and cup holders.