In the film, Harry Callahan, a San Francisco Police Inspector, drives a white 1971 Dodge Charger as he hunts down a serial killer known as the "Scorpio Killer".
There are a few other films where a white Charger features prominently, but the one that most closely matches your description is "Dirty Harry".
Do you have any other details about the movie you are thinking of? Perhaps the plot, a specific scene, or the name of an actor involved?