1985 Mercedes Benz 300sd Heater not getting warm enough

Our Mercedes Benz 300sd Turbo Diesel is very slow to warm up the car on the inside. We live in Michigan and always warm up the car approx 15 minutes before we go anywhere. When the heater is set on E/C it will warm up after quite some time, but it often fogs up the inside of the car, so we use the recirculate buttons instead. These functions keep the car from fogging up but just doesn't warm up the car inside very much at all. The engine temp. Rarely goes about 40ish. Any suggestions? I was wondering if the coolant leve effects the air temperature in the car. Would sure like to be warmer in the car, without the fog. Any suggestions?