37th

So a couple decided to spend the weekend driving up from LA to San Jose. Along the way their car runs out of gas, real smart huh? The husband, he leaves the wife in the car to go find a pump. About a half hour later, the guy comes back with a gallon of gasoline but inside their car, the wife is dead. She's sitting in the passenger seat with a big old hunting knife sticking out of her chest. All the doors are still locked, the windows are up, none of them are broken. The car doesn't have a scratch, it looks the same exact way it was when he left it. How did the murderer kill the wife?