Complex sentence: By 1990 "Bronco" proved that they were gaining fame because that same year they were the protagonist of the movie called "Bronco la pelicula" in which all four members of the band played a role in this film.
Split into simpler sentences: By 1990 "Bronco" proved that they were gaining fame because that same year they were the protagonist of the movie called "Bronco la pelicula". Which all four members of the band played a role in this film.