I watched a movie a little while ago, it was from the early 80s or late 90s and it had 2 girls, one blonde one redhead, and there was a lot of sex in the very beginning, in a car. with 2 guys they met at the beach or a gas station or something
A "beach", "a gas station", or "something", do you recall if it was the grassy knoll at Dealey Plaza?
Maybe, the movie was the true crime inspired "Bully (2001).
Rachel Miner had red hair and Bijou Phillips had blonde hair. One of the many sex scenes in the movie occurs near the beginning of the film. That scene is in a car after a day at the beach.
Yet another possibililty is Riding in Cars with Boys (2001) which I've only seen portions of, but which I believe stars a redhead and a blonde, and has scenes inside a car. Based on the pic below, I can't tell whether she's a "true" redhead ... but close enough.