Almost all the old cars especially those that are not luxurious, do not have a built-in stereo system in them. But since, if you have one that doesn't have that system, you should think about upgrading the car's interior with the double din head unit. There are two main types of head units, the single din head unit, and the double din head unit. But double din head units are not like the single din units. In fact, they have bigger screens, additional features, and the ability to make a great look of your car's interior.