I have just recently done this on a '85 300D by connecting the vacuum supply straight to the vacuum pod for the center vents. Works good and you don't have to worry about the center vents not opening. If you don't want air blowing out the center vents just close them off. I am not familiar enough with your car to tell you whether this will work or not but if the center vents are vacuum to open then it should be doable.

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Jim