Jim, have you looked at the wiring diagram? The ETM wiring diagram also has the vacuum flow and the theoretical airflow shown. Do you have the CD for this? I don't so I'm not sure which drawings they use.
One thing I can say is that the Center Flap Switchover Valve (code 3) when energized causes the center ducts to open and the defrost to close (partially). The defrost flaps switchover should also be energized to have no defrost (it seems). The problems here are almost never electrical. they are almost always bad vacuum diaphrams in the vacuum elements. If either the defrost or the center vent are leaking the result will be what you have as they are applied with one source.
Bosch Master, ASE Master, L1
33 years MB technician