My 2003 CLK320 heater only blows lukewarm air on cold days. Another thing it's been doing lately is that when I'm stuck in traffic, the temp gage creeps up and the heater air gets even colder. If I put it in neutral and rev it up a bit, the heater air immediately gets warmer and the temp creeps back down. Coolant level is OK. Water pump, maybe?

 

How about the thermostats?

For my 2004 CLK320, the water pump failed back in October-2011. When the dealership replaced the water pump, they replaced the thermostats as well.