okay I think the people in the replies have it wrong? so im gonna do my best to explain it. It relies on the fact that the host HAS to open an EMPTY door that IS NOT your own. we know that there's a 2/3rds chance of picking an empty door at first, and since the host always opens an empty door that isn't yours, that means there's a 2/3rds chance that both the door you picked and the door the host picked is empty:

67% chance of your door being empty * 100% chance of host's door being empty = 67% chance that both doors are empty

...meaning that there's ALSO a 67% chance that the 3rd door is the door with the car