Sorry it took till now to get back to you, but Iím Ďunder the weatherí. Plus Iíve been busy in another thread making bluntly truthful (yet non-politically correct) statements that will be perceived as insulting or inflammatory by some. (I'd put the smiley face with the big mouth and surprised look here if I knew how.)
RE: your auxiliary fan not functioning:
I looked at the wiring diagram for the 1979 240Ds, and lo & behold, there is a coolant sensing thermo switch that controls the auxiliary fan relay in addition to the one on the AC receiver/dryer. (Unlike my 84 300D, with *only* the AC receiver switch. WTH were they thinking?) They are both fed current by fuse #12, which should be an 8 amp fuse. I doubt that the fuse is your problem though, as it also controls both of the AC relays, and you said that your AC does work. As a side note, the temp switch on the receiver is supposed to come on at 52 C if I am reading the diagram correctly.
You may want to check the fuse that supplies power for the fan itself. If it is good, then bypass one of the temperature switches and check for voltage at the fan itself. If there is voltage, the fan is bad (assuming a clean ground, check that too). If there is not voltage with the switch bypassed, (and a good relay - the relay could also be bad in which case you won't get voltage at the fan) there is an open wire somewhere. (Remember that these tests require your key to be in the on position to supply juice to the system.)
I hope that this helps!