I believe your fan problem is the same one my '75 280 had... the motor is bad. The air conditioner uses a separate fan on those cars, and the fan for the heater is really buried up in the dash behind almost everything. Replacing it took me a couple of days on my 280, as I had to remove both front seats, the center console, move the air conditioning evaporator unit out of the way, remove the heater core and housing, and then I could see the fan. I got a used one and replaced it, after making sure the new/used one was well lubed. The old fan had fried it's windings because the bearings had seized up. If this sounds like a lot of work, it was, and since my car's heater worked as long as the car was moving, it never bothered me, but my wife insisted that it should work when the car was stationary. Before going to all this work, make sure that the fan does indeed have voltage to the motor! The air conditioning switch must be turned completely to the counter-clockwise position to allow the heater fan to come on. I could be way off base here, never having had a 240D.
'82 300D/4.3 V6