From my reading (and I'm no electrical expert having a ME background not EE) Mercedes uses current limiting to control motor speed. So there are some active electronics in the module - still no where near $200 of components.

On further reading it looks like the W140 (and early W210) blower regulator has the same functionality (albeit different wiring). It looks like these can be had for ~$30-150, what would be necessary is to cut their 4 pin connector and replace with a W124 harness. It looks like the W140 part's 4 pins are

1) Red (large gauge) - hot to motor
2) Red (small gage) - hot to regulator
3) Black - ground to regulator
4) yellow - variable volt to regulator

Output from regulator is one large gauge blue

So it looks like you could make your own harness for a w124 - 3 wire harness (really just splitting the red hot wire into two leads);
red - directly to fan and a tap off to the regulator
black and yellow go to regulator
and the blue output also goes to the fan motor

Anyone know if this will work - searches don't give any definitive answers that I could find.

Paul