A bumpersticker was a sticker attached to an automobile. They usually displayed a message of some type which reflected the views of the person who owned or operated the vehicle.
In 1986, Gillian Taylor's Chevrolet C/K truck had a bumper sticker which stated "I ♥ WHALES" on the rear window, which fit with her job as assistant director of the Cetacean Institute. (Star Trek IV: The Voyage Home)