How do I turn on the heater? on 1993 Mercedes-Benz 300SE

I have used the air conditioner with no problem, but the weather has turned cold. I tried turning the tempeture wheel to red in hopes that it would heat up, nor would the air to the front windshield heat up. Am I doing something wrong or is there a mechanical issue? The car was a gift from my boss and I misplaced the manuel.

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