You have the right info. In the snowflake position A/C isn't added to the equation untill all heat is over. It tells that by an electrical signal from mode door poteniometer. The pushbutton controller waits for a certain voltage (there are at least two versions at two different voltages). Thus the problem can be that the door doesn't make it closed or that the potentiometer is bad or out of adjustment.
You will need a vacuum schematic to trace the vacuum leak if such is the case. The CD covers this fairly well.
Bosch Master, ASE Master, L1
33 years MB technician