I would never leave a dog in a car unattended even if the weather is good. Too many ways to get hurt by falling not to mention excessive barking/anxiety can lead to problems. I'm a retired police officer and nothing made me madder than finding a child or dog who'd been left in a car. It doesn't take long for a child or dog to overheat even if it isn't extremely hot...the combination of a little heat and anxious barking/panting/crying can lead to overheating in a matter of minutes.
Well, they shouldn't be ABLE to fall, since they should either be confined in a crate or restrained in a car harness.
And as far as barking and anxiety are concerned, that's a training issue.
MANY, if not most people who do trials and dog shows leave their dogs in the car at least some of the time. The dogs usually rest better and are more comfortable in their own surroundings. HOWEVER, we learn how to set the car up to manage heat, we check the dogs regularly and we DON'T part in public places where theft is a danger.
So I am not totally against leaving a dog in the car, but it has to be managed well, with LOTS of air flow, shade, either from a shady parking space (which you check often to make sure the sun isn't shifting) or a shade blanket for the car. I have a Mini Cooper with a hatch back and a sun roof. With the windows, hatch and roof open, it is no hotter in the car than it is outdoors. AND it has to be in a secure location, where the dog is safe from theft. Finally, the owner has to train the dog SLOWLY and CAREFULLY that the car is a safe place to stay, and that the owner will be back for them soon. This part is no different than crate training.