It's a big issue with our cars, the belts don't like to roll back up into their houses. Like Ferocious said, don't unroll it while it's out of the car thinking you are helping it somehow. There's an enormous magnet in there (you'll feel it when you pull it off) that somehow triggers the lock mechanism to the belt. Out of the car, they don't roll back up, but they will unroll. Makes for a long afternoon of cursing.
Orange was right mostly, to pull the whole assembly you need both of the back panels out, but you do not need to pull the rear seat. If you do it might be a bit easier, but it's not really a hindrance.