In re-reading my owners manual, I learned the followingabout the climate control pushbuttons and how they change air flow from the center vent:
The button "black arrow up/white arrow down" is the recommended normal setting for cooling and humidity control. Warm air is given priority flow to the feet area, but also comes out the side and center dash vents, and a small amount to the windshield dash vents to keep it clear.
The button "white arrow up/white arrow down" is multi level for "clearing a fogged windshield-As soon as the windshield is cleared, reset to normal or EC mode". Warm air may only periodically come from center vents in this mode, depending on interior temperature. Also, "prolonged use of this mode may cause condensation on outside windshield in warm, humid climates". (It does!)
So it looks like the black up/white down button is the place to leave it. Try this button and see if that changes the air flow from the center vent.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E