I hear it's a relatively trivial replacement under/behind the glovebox, on a w123 - never owned one, so can't say from experience.
In Mercedes' defense, the heater fan on my '76 worked until this winter - 30 years. (I also hear, but have no direct experience, that the w108 chassis is the same way regarding the HVAC design.) Stuart S. 76 300D w115 Zoltan Finks said: This is the most disappointing thing I've heard yet about the engineering of these vehicles. Why such a blunder? Is this the worst thing about our Mercedes? The 123s are better on this I hope? One thing that makes me fond of a vehicle is the notion that every detail was intentionally designed with intelligence and for the good of the user. Brian 83 240D