A “2 DIN headunit” is simply the larger of the two form factors that almost every head unit comes with. If you’ve heard you need one, it’s probably just because that’s what your car has now, and swapping like for like is the easiest way to upgrade a car audio system.
How to Choose a Car Stereo | Crutchfield
If we look at it a little bit more, the two main radio sizes are “single DIN” and “double DIN”, and it’s actually quite easy to determine which one you need. If your car has a single DIN head unit, the faceplate should be approximately 7 x 2 inches (180 x 50mm).
If you have a double DIN head unit, the faceplate will be the same width but twice as tall. Since “2 DIN head unit” is a colloquial term for double DIN, the head unit in your car will be approximately 7 x 4 inches (180 x 100 mm) if it meets that standard.
The simple answer to your second question is that you never need a double DIN head unit. If your car came with a double DIN head unit, you have the option of replacing it with a single or double DIN radio.
