When slowing down or at a stop,it blows cool air.
Heater is set on high, heat do not get hot anymore(Warm air) on 1999 Mercedes-Benz CLK430
Check coolant level, is temp at running temp aprox 180 - 200deg. Water pump would be last option.
poss thermostat or coolant level
Blower resister no ebay no Chinese it's under glove compartment change fitter half hour work. Part $125 to 250
When engine is cold, and if you have a expansion tank system, check water at radiator cap. My guess is low coolant too. Could be if you have a bad cap or something is loose on line to expansion tank, coolant will not return to radiator. Also look for a small leak if radiator level is down.