While it should be common knowledge that you mustn't leave your dog (or other pet) in a hot car while you're away, lots of people do it. Please stop!
For some first-person evidence, watch this video in which veterinarian Dr. Ernie Ward closes himself into a parked car for thirty minutes. He even cracks all the windows beforehand, but it's still an utterly miserable situation. Can you guess how hot it'll get in there? Watch and learn:
Note: given the outdoor temperature (reported at the end) and the starting temperature in the car being roughly the same, I assume the car was air-conditioned and running until a few minutes before he started the video. So this does look like a "real-world" test.