If yes, for how long? Since childhood or only recently?
If no, why not?
Yeah, I'll instinctively put a seat belt on in any car, irrespective of where within it I'm sitting.
When I was kid, however, I very much remember spending longer journeys with my parents laid out on the back seat, either sleeping or playing games. That feels a bit mad to me now.
I also remember going to visit an ex-girlfriends family in Cyprus a few years back. They picked us up form the airport and were laughing to themselves when we both climbed into the back and put our seatbelts on. "Look at these Brits". They didn't wear theirs at all.
Thinking about it, I never think to put one on if I'm on a bus or a coach though, even if they very obviously have one. Which is strange, given I do it naturally in a car.