Yes, but, I probably do automatically because it's always been the law for my entire life and I've never experienced a "norm" where people don't wear them. It feels risky to me not to wear one, as opposed to feeling extra-safe to wear one.
Is the scenario - "the law gets vanquished tomorrow"? Because I'd stick to my same behaviour, and I think a lot of people would because they probably feel similarly to me in that wearing a seatbelt is normal, not wearing one is risky.
If the scenario is - "they were never made law in the first place" - I don't know if I'd be so sure of my answer, and I don't think everyone else would be either. Before seatbelts were made law in the UK, observed seatbelt use was very low despite safety campaigns, and I don't think people in the 70s were fundamentally different to today. In fact the cars had fewer other safety features and fatal car accidents were much more common, but it didn't seem to make much difference to people's behaviour or beliefs about how useful seatbelts are.
You see the same pattern happen again with children's car seats and you can even go back and see BBC news articles about it in the late 90s where they tried lots of safety campaigns to get people to use booster seats for primary aged children. The norm at that time was for babies and toddlers to be in car seats but older children just used seatbelts. A minority of parents used booster seats, usually to help children see out of the window rather than for safety purposes. Eventually the law was changed in 2003 to say children aged up to 12 or a certain height must continue to use a car seat or booster seat. People thought it was madness and now it's just normal, and you'd be thought shockingly lax to drive a 5 year old around in just a seatbelt (let alone with none!)
I think it's to do with how our brains tend to perceive risk - the reality is, we get into a car thousands of times and travel and nothing bad happens. If you're not wearing a seatbelt, that gets imprinted as the "norm". You have to have some kind of external force like a law in order to change people's behaviour and therefore create social pressure to the point where the majority now see seatbelt use AS the norm.
I think if the law got vanquished tomorrow, you'd see an immediate drop in usage as people who really dislike seatbelts stop using them, but rates would remain higher than they were pre-law for a while, but over time, you'd get people slip from "I must wear it every time" to "Oh this is just a short trip/nobody else is bothering" and because the vast majority of the time nothing bad happens it reinforces that as a norm in your head and the risk starts feeling less serious, so I think you'd see seatbelt use drop over time, but lots of news reports about people dying and whether or not they were wearing seatbelts (which would probably just confuse the average person's understanding of whether or not they matter).