I personally would never leave my dogs alone ANYWHERE, besides in our own home or in a home of our friend's.

Being in Florida there's no way a dog would survive in the car with no AC. If we bring one of the dogs on a car ride to go to the store, I stay in the car with them, WITH the AC on.
Besides getting heat stroke, who knows if someone's going to tease them, steal them, etc. If the windows down, slip them something dangerous to eat. So many things could happen and I prefer to take that extra step to make sure they are safe. There's so many evil people out there who are willing to do all of the above without any guilt on their mind whatsoever.