On the first gen W126, under the booster is where the blower fan resistor is located in addition to the aux fan resistor. Believe me, I spent YEARS trying to find out why the hell I had only high speed blower fan. PBUs and both relays swapped making myself nuts.
PO had a small melt down in there, the aux fan still worked on low (shockingly) but the wiring was very crispy. I spent quite some time in there with a soldering iron, heat shrink, and quality wrap.
This is the same reason my car had no ABS working, LF sending wire was bad. I have dirty exciter rings I believe, but hey... at least the system goes out of fault mode once the vehicle starts.
I believe MB moved the blower fan resistors beside the actual blower on the second gen W126. Seems to stick in my mind as there.
My 300SEL had a broken wire to the fan resistor and the fan was jumped to run high speed only for years. Worked fine just as your diagram shows. I fixed it and now use low again.
I missed the 'aux fan, high speed only' detail in your initial post. Sorry about that!
