Well, I don't keep this all in memory so I had to wait till I could look it up and be sure.
The water valve vacuum comes from the switchover valve (elctrically operated vacuum switch). It is part of the switchover valve group. Source vacuum (for switching) comes from the engine and I didn't look it up.
The decision to switch vacuum to the heater valve and the electrical control of the switchover valve is done in the pushbutton controller based upon various inputs: evap temp, outside temp, feedback pot, and incar temp.
I would check your source vacuum to the switchover valve group.
Bosch Master, ASE Master, L1
33 years MB technician