The way the fan speed is reduced to have a low and high speed in early cars without electronic control is with a Ballast resister. I am not sure where the term comes from. Most have only heard of its similar use with ignition coils. Maybe the term only applies to resistance added to ignition. This resistance is wrong for ignition coils I have seen but standard ballast resistors on MB ignitions have these resistances: 0.4ohm, 0.6ohm, and 0.9ohms. The ballast resistor on most MB fans looks identical to these other ballast resistors but has a resistance of 0.2ohms.
I have even seen these resistors catch fire on 126 cars. The resistor is beneath the brake booster in that hole that fills with leaves. Once dried sutiably they can be lit off buy the heat.
Bosch Master, ASE Master, L1
33 years MB technician