the car is a metaphor for their relationship. At the beginning, Childish is in control of the relationship (which is why he’s driving the car while she’s in the backseat). When he kisses her, he gets attached to her and allows himself to love her. At that point she gets the upper hand in the relationship (which is why she starts driving the car.) In the end, she leaves him broken hearted and alone in the car while she walks back to her house.