If you mean the heater blower fan, it won't kick in until the coolant has reached preset temp., which could mean you're in a cool climate and it takes longer (my 380SE can go 2 blocks this time of year before the fan starts), or there's a problem in the circuitry of the climate control, including the controls for this function. You can probably rule out the blower itself if the fan comes on high when the button for defrost is pressed (assuming you have auto system).

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1986 560SL
2002 Toyota Camry
1993 Lexus