"in the back" always implies where the person is waiting (back of the building, back seat of the car, etc) while "behind" needs a clearer object (behind the building, behind the car). So if you say "waiting in the back for a car" you can assume the back of the building/lot/room but you wouldn't say "waiting behind for a car".
You might if you were trying to say that one of your party remained waiting for the car while the rest decided to walk on.
I put "He's waiting behind a car", but that was wrong... so would that be "Ő vár hátul egy autót"?
"Behind" is simply "Mögött" - "He's waiting behind a car" would be "(Ő) Egy autó mögött vár".
What does in back mean? I am australian and we don't have that terminology I don't think.
The sentance is a very unlikely one in English......is the intention to say something like "She is in the back (ie, yard or something) waiting for a car"?