'89 300E no heat out of passenger side

Hello all,
I have been struggling with a strange problem in my '89 300e. When setting the climate control to heater (floor and Defrost simultaneous),
I get heat out of the following vents:
Drivers side dash vent, floor, windshield, rear driver side floor Passenger side dash vent and windshield.
I cannot get any heat out of the front or rear floor vents on the passenger side (thusly, my wife freezes). Everything works as it should (i.e. all settings for the climate control) except for the passenger floor heating. I have removed the glove box to observer the movement of the vacuum control for the system, and it looks to be working fine.
Anybody have any idea's
Warm Thom, cold wife.
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