I think this is correct, you usually get a leakage of outside air from the center vents when its in heater operation. I think if you really crank up the temperature it will eventually send warm air through the center vents.

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Dennis
2006 E320 CDI Black/Black Leather. Nav, HTD Seats HK stereo 32000 miles.
2000 VW Passat 1.8t 300,000 miles as of Dec 09, 2009 original owner
1997 E300d Silver 300,000 miles As of Jan 05, 2010
3rd owner SOLD