http://www.snopes.com/horrors/madmen/backseat.htm

Origins: _ This legend first appeared in 1967 and quickly caught on, becoming one of the favorite scary legends of that period. In addition to circulating orally, it showed up in Ann Landers' column in 1982, presented as a harrowing experience that had befallen the letter writer's friend.

Despite the legend's many incarnations and long history, there's no record of its ever coming true. It's a cautionary tale, warning us to be vigilant of our surroundings.

There just aren't that many bad guys lurking in backseats to get worried about. Though there have been rapes wherein the attacker hid in the back seat of an automobile, they can be counted on the fingers of one hand. As for carjackings, in the overwhelming majority of them the assailant opens the door and gets into the car while the driver is behind the wheel. There's little of this "lurking in the backseat" bit.

The legend might have been prompted by a real news story, though:

[Drake, 1968]

Research shows that one true case of "The Killer in the Backseat" did occur in 1964 in New York City, when an escaped murderer hid in the backseat of a car. The car, ironically, belonged to a police detective who shot the man. Though the differences between the legend and the true occurrence are vast (the real occurrence did not feature a lone female; it didn't necessarily happen at night; and no third person was involved), the legend may have sprung out of this real incident.