The plot of the movie was a guy buy's this really nice sports car from this hot sales girl and all the cars are guted and junk parts are put in. And at the end she ends up getting him a new car, and I think they destroy the dealership

Pretty sure you're thinking of 1988's It Takes Two (http://www.imdb.com/title/tt0095384/), written by Richard Christian Matheson (son of famous genre specialist Richard Matheson) and starring a pre-24 Leslie Hope.