This is not an MB specific reply, since I never paid attention to whether my MB's work the same way as my other vehicles in this regard, but it might shed some light. If the MB is like other cars, one of the "heavy load" items that comes on when you switch the heater lever to the "defrost" position is the A/C compressor so that the air going to the windshield is de-humidified by the A/C system regardless of whether you actually want any A/C (meaning cooler) air at that time. If you're really just using the heater core and blower to augment the radiator in a situation where the motor would otherwise overheat, you'll probably find it works just as well to set the heater output to the floor or the center vents instead of defrost to avoid toggling on the A/C compressor with its additional electrical and mechanical load. Of course, if the engine heats up in traffic and you have to run the heater to keep it out of the red, there's probably something else going on worth looking at.