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.

Asked by for the 1993 Mercedes-Benz 300SE