If its like the 124 model with a row of square buttons, a temp dial, and 3 fan speeds, it should work like this: Defrost button automatically operates maximum heat regardless of temp dial setting. Heat to windshield and outer vents. Up-down button splits heat to windshield, outer vents, and floor vents. Temp and speed dependent on dial and fan buttons. Center button (most commonly used) supplies heated air to outer vents, floors, and a slight amount to windshield. Heat is only supplied if interior is cooler that temp dial setting or dial is set to full heat. Cooled air on this setting blows out all four front vents and a slight amount to windshield. Cooled air is only supplied if interior is warmer than temp dial setting or if dial is set to maximum cool. EC button works exactly like the button just to the left of it, except the AC compressor is not turned on. Air is exterior air. 0 button allows some convection heat.

Hope this is helpful.

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2002 Mercedes-Benz E320 4Matic
1995 Mercedes-Benz E320 Wagon
2005 Land Rover LR3 V8 SE
1999 Audi A8 4.2L quattro