PS...Don't do the $50 solution mentioned above. It's NOT worth it.
Actually, the $50 solution might work better on a Mercedes Benz than many other cars. Here is why:
Back in the 1930's, an engineer, Nils Eric Wahlberg at Nash motor company did extensive wind tunnel tests. He found that when a car was in motion, the pressure inside the car was less than the pressure outside the car. With all the ways that cold air could infiltrate the cabin, he devised the fresh air heater that brought in air from the outside rather than reheat the air in the cabin and recirculate it. Bringing in the fresh air pressurized the cabin. This system became the Nash Weather Eye heating and ventilating system which Consumer Reports said when comparing the Nash Weather Eye to the old system was like comparing a modern forced air furnace to a coal heating stove. Eventually, all cars adopted the fresh air system. The last car that I remember using a recirculating heater was the 1957-8 Studebaker Scotsman. Studebaker wanted to keep things simple on this car. Since the Mercedes Benz has a better fitting body than many other cars, the $50 solution would probably work. It would, however, look like hell.
As far as a fan to defrost the windshield, I have a slant back 1978 Oldsmobile Cutlass Salon. It didn't come with a rear window defroster. I tried a kit where a grid of wires is cemented onto the window but it didn't hold up. I finally mounted a small fan on the package shelf and it defrosts the rear window very well. The same solution would work for defrosting the windshield as well. In this old Oldsmobile, the heater core replacement was much simpler than my $50 solution. The heater core and air conditioning evaporator are in a box under the hood. The heater core replacement was a 30 minute job. Once in a while, General Motors does do something right.