In this case, the defendant would be guilty of larceny. The fact that the door was open and the keys were in the car is no defense whatsoever.
The crime of larceny is defined as the unauthorized taking of someone's property with the intent to permanently deprive the owner of its use. It does not say that the accused must go to any great pains to take the object. The fact that the defendant had no authorization to take the car means that he is guilty, regardless of the fact that the car was easy to take.
So, as long as the defendant did not have permission and as long as he intended to take the car away, he is guilt of larceny.