Right, if any of the two heat control knobs are not in fully cold
position, the vacuum valve opens and provides flow to the common heater
core. Each heat control knob then controls a flap, which mixes hot air
from the heater core with cold fresh (or recirculated) air according to
the position of the control knob.
In my 1987 190E, the knobs and flaps are connected by a mechanical
cable. This should be easy to troubleshoot (but difficult to access).