I think you just need a "4 wire" pot.

However, if it's any kind of standard electronics pot, it probably doesn't have the capacity to soak up the load from the fan motor. Does the w124 have electronically controlled variable fan speed? If so, you could probably use a small pot and splice it into the electronics to bias the speed up or down as desired. You'd have to actually go into the CCU though and be pretty savvy with circuits to determine where to put it.

If the car doesn't have electronic controls, you'll have to build a chopper for the DC with a variable frequency to run the fan.

Failing both of those, you'd need a giant pot with ceramic insulation to handle the heat, spliced into the leads for the fan.

__________________
-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250