This one tends to have befallen the sister of a friend of someone you met on MySpace. The true urban legend is that anyone ever met anyone real on MySpace, but will require its own entry. For now, let us concern with the Sister of Dubious Origin and the man in her backseat.
She was driving home – from work, from a party, from college, it really varies based on the Sister of Dubious Origin, and has befallen one or another of them at some time. What's constant in every story is the truck that began following her. At first it only flashed its high beams, but seconds later it was tailgating. When she slowed down, it refused to pass. When she sped up, so did it.
She tried pulling onto back roads, but the truck continued following her, even nudging her rear bumper as its horn blared. In some versions she called the police on her cell; in others, a parent or boyfriend. The truck always followed this sister in every version, to her apartment, to her parents' home, or even a police roadblock. As soon as she jumped out of her car, so did the trucker, screaming that there was a man in her backseat.
She whirled and saw that, yes, there was a fastidious and photogenic man leering from the backseats. Every Sister of Dubious Origin who ever lived heard him ask, "Did you know you can save 15% on your car insurance by switching to GEICO?"