Mine fried as a result of :
W124 Climate Control Mystery
Voltage was correct at regulator from push button control box, but fan was on high at all times. The new regulator is not cheap, but it does include the heatsink and is easy to install once you remove the wiper motor.
1987 300D (176,000 mi)
2001 CLK 430 Convertible (18,000 mi)