I mounted my 2 x 10" subs in the sides of the boot, you know the area to the rear of the car, before the wheel arches. I have also mounted my amps and changers there. I can still get the buggy in and out of the car, and also the spare wheel.
I aint got any pics at the mo, will post when i have.
i've acquired some mk5 SRi astra seats, i've compared it to a mk4 seat and the subframes are the exact same, infact the whole seat is almost the exact same apart from some minor changes.
one of the differences is the wire from under the seat, but it is just 2 wires going into the connector...