A car becomes iconic when it gets seared into people's brains. When someone sees a similar car, do they immediately think of that movie? If yes, then the car became iconic. Some examples of cars that became iconic to me: 1958 Plymouth Fury (Christine), 1958 Chevy Impala (Peggy Sue Got Married, American Graffiti--probably only iconic to me cause I love that model car), 1959 Cadillac Miller-Meteor (Ghostbusters), 1974 Dodge Monaco sedan if painted like police car (Blues Brothers), 1970 Chevy Nova (but only if blue and crappy looking)(Beverly Hills Cop), 1976 AMC Pacer (Wayne's World). I'm sure there are others, but those are some I can think of.