This is really important. Dog’s shed heat by panting (instead of sweating) - they expel moisture and heat as they pant and if that humid air stays around them, they will no longer cool down but will instead heat up even more, and become dehydrated at their body tries even harder to get rid of heat through panting.
A dog can get heatstroke out in the open if they are panting too heavily and making the air around them too humid and stuffy - they need to be moved (if possible). If that can happen to a dog sitting out in the open think about how much worse it would be within an enclose space like a car. Hyperthermia will kill a dog and it is a terrible way to go.
dont leave your dog in your car you pricks
I have had the misfortune of seeing a dead dog inside of a car one summer. This is a crime, please do not let this slid if you see it.