"Mustang cars" sounds okay, Neostudent. I can't imagine using the possessive for any brand of car. Most of the time, people simply use "Mustangs" to refer to that brand of car: He really wants a new Mustang.
"Mustang" is not a company, it's just a brand of car. It would be like saying "a Persian's cat," which means any cat owned by a Persian, when you mean "Persian cat," meaning the breed of cat called "Persian." "Mustang's cars" would, I guess, mean a group of cars owned by a horse.