Is it against the Law in South Carolina to leave a pet in a parked car?

Check with South Carolina DMV.
If it is not against the law it very well should be. Leaving the dog in the car is just asking for trouble, if it is hot your dog could be overcome by heat exhaustion, if it is cold (i doubt in SC) your dog can freeze. If you leave the window open your dog could escape and run away, or even worse bite someone. The bottom line is you should not leave your dog unatteded in a vehicle for more than just a couple of minutes.
Whether or not it s against the law, why don't you consider this - would you want to be left in the car for whatever period of time you plan to leave the dog there, under those conditions. If you know you would not want to sit in a parked car, in the heat, without the air conditioning on for 'only 15 minutes' do not do it to the dog. If you think it would be too cold for you to sit in the car, it is too cold to leave the dog there.
In CA the law differs in each county. Sometimes cities have local ordinances as well. You would call your County Animal Control or your local ASPCA. It would have nothing to do with the DMV.

Amazingly, it does not seem to be against the law.

However, if the animal died as a result of being left in a hot or cold car for too long, the state's animal cruelty laws would apply.

Common sense should prevail and anyone should realize that it can VERY quickly become too hot for animals, even on an overcast or cloudy day. The animals suffer a horrendous death breating in superheated aur and becoming dehydrated quickly.

Never leave your dog in a closed car, ever.
In most state's, it's only against the law if the pet is in danger due to heat or extreme cold. But it is just common sense, although not everyone has common sense, that if it's above 60 degrees or below 40, don't leave your animal unattended in a car. An animal can die from the heat in a relatively short time in a closed up car. It's like an oven. If you want to understand how your pet feels, try sitting in your car with the windows up and the engine off for a couple of hours in the sun without water when it's over 60. Then you'll see why it's a very, very, very, bad idea.......