Yes correct thats the cavity behind the rear seat. I think sometimes they have subs in there?
They are Vertex batteries. http://batterydirect.co.nz/Race%20cars.htm
Man I would love to make my Supra V8 turbo a 4WD with this system... now thats a unique car!
Nice work mate... one of those projects you start and conceptually all sweet, then doing it a PITA?