We have a Toyota Emina 7 seater, which has 2 front (driver/passenger), 2 middle, and 3 back seats. We've put bolts behind the 2 middle seats - attached to the base of the seat itself, inside the plastic housing. My parents had the car before us, and they had a bolt attached to the base of one of the rear seats for Kiya's carseat, again on the seat itself, so that when the rear seats were folded up it wasn't in the way.
Does that make sense?

We can have 4 bolts in total if we want: on the two middle seats, and two on the rear seats (just not one for the back centre seat)