Found it! One of many versions, anyway:
This is a classic urban legend, dating back at least to the 1940s. The familiar tale is most often told as having happened to a friend's college roommate or high school classmate, and the story of the murderer's hook on the car door has long been a favorite at slumber parties and around campfires. http://www.unexplainedstuff.com/Superstitions-Strange-Customs-Taboos-and-Urban-Legends/Urban-Legends-and-Beliefs-The-hook-on-the-car-door.html