I've always heard it should be around 20 degrees cooler then the outside air? You'll be happy to know, I was out side working on my car in a T shirt yesterday afternoon sweating. Then I drove around with the windows down. I feel for you, really, I do.
My XP with Automotive a/c is it is anywhere from 20-40 below ambient temp. Depends on system and type of refrigerant. While I have never seen this condition of a frozen Heater core, in an older car it is a likely scenario. Newer cars the a/c heater box is built to keep them shrouded from each other.
