I'm of an age(early 30s) where seat belts were drilled in-both by parents and at school-from the time we could comprehend them. I vaguely remember by grandfather complaining about being "made" to wear a seatbelt after he got an additional ticket for not wearing one, and I think around the time I started driving(maybe a little before) no belt went from a secondary offense to a primary offense in my state.
Putting the belt on is an automatic action for me when I get in the car, whether the front or back seat.
In fact, my MG has early non-inertia 3-point belts. They also have an interesting(Kangol brand) magnetic latch that's super easy both to attach and take apart, but very secure once buttoned. In any case, the shoulder straps attach to posts on the body in front of the deck lid. When in place, the shoulder belts will block the top from raising and lowering, so there's a quick attach/detach mechanism via a plastic "keeper." The plastic keepers are often missing or broken, but are integral to the belt functioning properly. When I bought the car, the guy who sold it to me said "I never bother with the seat belts-I never could figure out how to work them." I had them at least in place before moving the car an inch(it took a bit of fiddling to figure them out) and had a set of the plastic keepers on hand a few days later. That's how strongly I feel about them.