Does anybody know what resistors are used for which fan amounts in the w116 (low/high/def)? I can't make it out in the wiring diagram.

The higher the indicated resistance the slower the speed. This may or may not help. What is your primary problem?

I was considering replacing the factory HVAC fan switching. Thanks anyway. I suppose I could find them and measure them!

I just threw that in as I do not have a good schematic for your car. Plus I was not certain what exactly you were trying to do. Perhaps someone that does will post yet.
Give it a little time. Also there might be a good readable schematic posted in the archives or on a site containing the complete manual for your car.