1993 mercedes benz 190e blows out cold/luke warm air! If im let warm up

I have a problem with the heater. Now that it is winter i am freezing my **** off with no heater. i get freezing cold air coming from vents on the dash(vents are in front of your face where steering wheel and radio is)those vents blow out ice cold air high. i bought the car and it was stuck on high and i can't get it to go any lower. If i let the car warm up for 10-15 minutes and then turn on my defrosters on(high Level) It starts out blowing hot air. and it becomes luke warm and then finaly blows cold air. what could it possibily be?