Steve,
I have a 1980 300D also. I've almost got the wiring diagram in my head from memory after troubleshooting so many things on it. The aux fan has one temp switch on the a/c receiver/dryer that activates it. My experience is that without running the a/c, it is VERY hard to get a properly tuned 617.912 non-turbo into the 100+ C range on the dash gauge.

You can verify fan operations by shorting the two leads on the recvr/dryer temp switch together. The fan should come on if the ignition switch is on. If not, go to the relay box on the left fender and check your "ice-cube relay" by switching it with another. You can hear it pick up if you short out the temp switch leads with the engine off and the key on. If the relay is picking up, you may have a melted brush assembly, the leading cause of death on aux fans of that vintage.

I'm trying to piece together a working fan from parts of several for just this reason. If anybody has a spare aux fan that runs, let me know if you'd be willing to deal on it.

Mike