I'm not 100% sure on dimensions but I'll have a measure up soon
From memory you've got about 8" width and about 4" depth but don't quote me on that until I've checked.
My local ICE specialist says he'll be able to build a pretty decent install with a small sub, possibly 8" in the space. Won't be earth shattering, but that's not what I'm looking for.
It was really easy;
> Put the top half of the back seats down by pulling yer knob
> Remove the carpet and board from the back of the boot
> The carpet on the back of the seats is held in by 4 rawlplug type things, use a small screwdriver to push in the middle and pull them out
> Remove carpet
> Take out about 6 12mm bolts holding the sub enclosure in
> Cut the wire on either side, there are a pair of wires in each insulator which will need re-connecting after you take the enclosure out
> Remove the enclosure and marvel at it's uselessness
> Put two bolts back in either side, you'll see the ones I mean