Wind Chill

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Prior to Christmas break, an unnamed female student (Emily Blunt) at a Pennsylvania university uses the campus ride share board to find a ride home to Wilmington, Delaware on 23 December. She finds a ride, which turns out to be a male student (Ashton Holmes), also unnamed ("the guy" in the credits), who is also driving to his home in Wilmington. His car is old and in poor condition, with the trunk full of his possessions (he tells her he is being kicked out of his apartment). It soon becomes apparent that she is somewhat arrogant and anti-social and his attempts to start a conversation go nowhere. It also becomes apparent that he knows quite a lot about her and possibly that he does not actually live in Wilmington as he claims. He tells her that they have a philosophy class together, although she has never noticed him.
The pair stop at a rather insalubrious gas station, where the girl goes to the bathroom and is temporarily locked in when the door handle sticks and then comes off in her hand. After managing to escape, she berates her companion for not hearing her shouting for help. She becomes even more suspicious when she hears him asking the clerk for directions, although he claims he has driven the route many times. Soon after setting off again, he turns off the main highway down Route 606, a lonely snow-covered road through a wooded ravine which he claims is a short cut. She tells him to return to the main highway, but he refuses. Crosses, apparently on graves, can be seen on the slopes on either side of the road.
As night falls, while approaching a bridge, the pair see headlights coming towards them in the middle of the road. The oncoming driver does not slow down, causing the guy to swerve. Their car ends up half buried in a snow drift and they are unable to budge it. The guy observes that the oncoming car, which did not stop, has left no tire tracks in the snow. A radio news report warns that an incoming storm will bring snow and temperatures down to 30 degrees below zero.
The guy heads back up the road to try to reach the gas station. The girl, staying in the car, sees a muffled figure staggering down the road past the car. She follows it and calls out, but it ignores her. Soon afterwards, the guy returns, saying the gas station is closed. She does not think he has been gone long enough to get there and back. The two huddle back in the car, where the guy finally reveals that he has been watching the girl for some time and has a crush on her. He actually only lives ten minutes' drive from campus, but was sitting behind her in class and saw her texting about getting a ride share. He immediately put up the notice on the board offering a ride to Wilmington and she took the bait. He says he's not really stalking her, but just wanted to be alone with her as he likes her. She apparently believes him.
They notice a number of black-clad figures walking past the car and call after them, but they do not stop. Their faces are white and dead-looking. The guy follows them up the slope into a ruined building where they disappear. He finds frozen corpses half buried in the snow inside and also a newspaper, which he brings back to the car. Meanwhile, the girl sees the staggering figure again and runs after it. She gets close enough to touch it, burning her hand, and it turns round, revealing a bloated corpse. An eel falls from its mouth and slithers away.
They think help has arrived when a Pennsylvania Highway Patrol officer (Martin Donovan) knocks on their window. Oddly, he doesn't seem to understand their predicament, choosing instead to believe they were parking. Taking the girl from the car, he tries to get her into the back of his patrol car, which is clearly not a modern vehicle. When it becomes clear to her that he means her harm, she struggles to get away, but cannot. The guy appears behind the officer and hits him across the head with a tire iron. They awaken, wondering whether it was all a dream, to find that the guy has the tire iron frozen to his hands, which are frostbitten. It becomes apparent that he was badly injured in the crash, and he admits that he did not get far when he went for help before he started spitting up blood, but claimed he had got to the gas station and back in order not to worry her. The girl reads the headline on the newspaper he found in the ruined building. It is dated 23 December 1953 and tells of an accident in which a young couple and a police officer were killed when their cars crashed into a ravine on this road. Priests from a nearby home for retired Catholic priests gave them the last rites. They accept that the police officer is a ghost and the black-clad figures they have seen are the ghosts of the priests.
The police officer appears again several times, always heralded by Brenda Lee's "Rockin' Around the Christmas Tree" coming onto the car radio. Other ghosts also appear. The girl drifts in and out of consciousness, becoming in her dreams people the cop has killed. She realizes that she can call for help from the junction box at the top of a nearby telephone pole, as the phone from the guy's apartment is in the car. She does so, watched by the ghosts, although luckily the cop, apparently the only malevolent spirit, does not appear. She gets through on 911, but the line is bad and she is not sure that they have received her message. Dispirited, she returns to the car, only to find that the guy has died of his injuries.
Some time later, there is again a knock at the car window. Expecting it to be the ghostly cop, she instead finds that it is a snow-plow driver (Ned Bellamy), who appears to be real. He puts her into his truck and the guy's body on the back and calls in that he has found the reported accident using his radio. As they drive off, he tells her the story of the road. In the 1950s, a corrupt cop murdered people on this stretch of road and their bodies were never found. In 1953 he was drunk and ran a young couple off the road, but he also lost control and plunged into the ravine. Frequently, at around this time of year, people are found dead on this road. In 1963, the priests who had given the accident victims the last rites were found frozen to death in their beds. Then the pair see headlights approaching. The driver thinks it is more help arriving, but it is of course the ghostly police car, which runs them off the road and disappears. The truck is not stuck, but, despite the girl's pleas not to, the driver gets out to help the driver of the other car, which he assumes has gone down the ravine. The girl follows him and the pair see two burning cars down the slope. The cop is trapped in his burning car. As they watch, the priests come down the slope, but instead of helping him they pull the microphone from his radio, preventing him from calling for help. They leave again as he burns to death. His burned body crawls from the car and touches the snow plow driver, who freezes to death as the girl watches. She tries to start the truck, but the policeman appears again. The ghost of the guy also appears and once again hits him with a tire iron, saving her.
As dawn rises, she awakes back in the car, with the guy's body next to her. There is no sign of the truck. The guy's ghost appears and leads her up the hill, through the ruined priests' home and finally to the gas station on the highway, where he disappears. She calls the emergency services.

Question: What part of the car does the snow-plow driver knock on?
Answer: window