My 124 manual says that in "Defrost" mode the blower will go to FULL, the heat will go to FULL and only the defrost vents will be open. This is to quickly melt any ice or frost on the windshield.

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1993 W124 300D 2.5L Turbo, OM602.962
2000 Chevrolet Cavalier, 2.4L DOHC
2002 Ford Explorer, 4.0L SOHC
2005 Toyota Prius, 1.5L

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