The "Killer in the Backseat" story plays on our fears that danger is right under our noses and we are not aware of it. This story was conceived in the late 1960's and has been circulated ever since. Some more recent versions of this story involve gang members out on initiation night, others involve modern gas stations. The story here is in a more traditional form and goes something like this...
On a moonless, foggy night, my friend was sleepy and having trouble driving on the road due to a recent rain. She was getting frustrated because she was running low on gas and there was no sign of civilization anywhere. Not knowing where the next town was, she was ready to pull over and get some sleep when she noticed a tiny, rundown gas station.
She pulled in and asked for the attendant to fill up her tank. She noticed he was acting very odd and somewhat irritated. He finally proceeded to fuel her car and then told her to open up the hood because "something didn't sound right".
Reluctant and afraid of the odd-acting attendant, she didn't want to be stranded later on down the road, so she got out. When she neared the front of the car, the attendant grabbed her and said she would have to step inside. With his hand over her mouth silencing her, he dragged her into the station.
Very afraid, she struggled, and the attendant let her go and blocked the door. He then said "There is a man crouched down in the backseat of your car and I didn't want him to know that I'd seen him".
After calling the police, my friend then learned that the stranger in her backseat was a serial killer who stowed away in her back seat, planning to make her his next victim.
This version sometimes has the woman and the gas station attendant getting married and having children. Other versions have the attendant telling her there was a problem with her credit card and that she'd have to come inside.
Yet another variation has a man getting into her backseat AT the gas station while the woman is inside the store. The attendant notices and tells the woman to stay inside and proceeds to call the police.
This is only an Urban Legend but there's a lesson here - While it's unlikely you'll have any serial killers lurking in your backseat, always keep your door locked and check the backseat before you get in.