D.S.
T.,
I know this is going to be way to harsh for some of the mama's out there but it works. I had a friend when I was a teen who couldn't remember to lock her car. It wasn't a big deal where we lived (middle of nowhere!) but when we got out of highschool & went to "the city" they worried that something might happen to her. The solution? They got a very close friend of the family to hid in the backseat of the car with a mask on and a toy gun (looked real). He "kidnapped" her, made her drive him to another location & got out. It scared some sense into her & made her realize what could happen. Her parents eventually told her that they had arranged it and yeah she was upset for a bit but knew that by them doing that it probably saved her from the real thing.
Not trying to convince you to do this but I think whatever you do needs to be drastic. For her safety & well being. Good luck!