Mandy was 16 years old the first time she let a heroin dealer into her car. The dark-haired cheerleader had left school early that afternoon in order to drive to a predetermined spot in the San Fernando Valley, an affluent area just north of Los Angeles made famous by Valley girl accents. Accompanied by a boy from her school who was one year older, Mandy parked on a tree-lined suburban street and waited. After 40 minutes, another car pulled up, and a bald, tattooed, mustachioed man got out. “That’s him,” said Mandy’s friend, who provided the hook-up. “Unlock your doors.” She did. The man opened the door to Mandy’s backseat…