I would start off with the simple things like ensuring there is sufficient coolant in the system and no coolant leaks. Changing the thermostat is also a relatively inexpensive fix that you could try. Does the car run cool and is the operating temperature as per the gauge normal? You could have a defective auxiliary water pump or monovalve. There may be a fault within the controls for the climate control system.
300SD Heating Problem, HELP (I'm Cold)