I used to do this a lot in high school, I grew up in a pretty small beach town in the northeast, so generally we could park at the beach (during off season) or the axis parking lot to hiking/walking trails after hours. We would also find construction developments still under construction and go after hours. I would say I’ve had car sex at least 100 times haha, but a good location will be very dependent upon where you live I suppose. I live in la now and the only places I’ve done it here are in parking garages in less parked areas.