Hi,
My rheostat is without function too. But in my opinion it's not necessary because it brings no light even in total on position. The bulb power is too low.
I had also bad experiences with fixed rheostats, my old /8 nearly burned down because it heated up.
I by-passed mine too, see att. picture, just bridge pin 1 and 2 of plug for instrument cluster.
The price for a new rheostat at MB actually is 160€, much too high for more or less nothing.
...WRe