There was no "factory ac" for the 912E. It was all a somewhat unique VPC setup that was installed at dealers. Don't expect AC at all like a new car but it may get you through an otherwise unbearable summer.
Here's the original instructions for installation:
http://www.texas912.org/912E%20Tech%20Data/COOL%201976%20Porsche%20912%20ZPW%20203%20912.pdf