It is almost impossible to work on that system without an appropriate vacuum schematic diagram.
I can tell you that it almost never is the pushbuttons or the vacuum switches or the vac switchover valves (unless they leak vacuum).
The system uses the vacuum switches (not the switchover valves) to shut down the blower and compressor if vacuum isn't on the respective switch. This is probably your problem as the defrost position over-rides the vac switch and lets the blower work in this safety related default possition. All vacuum servos (vac elements) default to the defrost position also without vacuum.
Check that you have vacuum on both lines going to the temp switch at the base of the servo. This switch keeps vacuum (and thus blower) from happening till there is warm water. One line is the source and the other goes to the push buttons and the top of the servo and the vac switch. You will not have blower until there is vacuum in both lines.
If you don't have vacuum to the switch I would individually check each vacuum element first and plug each line going to a leaker. When those leaks are by-passed the system will work (most likely), it just won't blow in the right places.
Bosch Master, ASE Master, L1
33 years MB technician