Maybe this is just me, or the group I'm with, but I notice that most people don't have the habit of wearing seatbelts at the rear seats of sedans. The driver and the one beside wear those belts, yes.
I can't think of the last time I was riding in the rear seat of a vehicle. Very rarely do I go anywhere with a group of people and, when I do, I'm usually the one driving. There's not a lot of people I trust driving me around.
I always wear my seatbelt, unless I temporarily take it off to reach for something. I don't feel comfortable (or safe) unless I'm "tied in". Even when I lay down on the back seat, I'll put the belt on loosely then lay down with it still on.
I usually wear it if I am travelling in the back of a car. I really don't like the lap belts that some cars have for the middle seat though. I am pretty sure they are not much good if the car stops suddenly.
Of course most people here drive cars, so the safety consciousness is high. I think I just need to be very particular about this when letting people in at the back seat. I'll develop this "please-wear-the-seatbelts" look when looking at them through the rear view mirror.