You have to replace the entire module, it's encapsulated in epoxy. If the fan motor were in poor condition, the brushes arcing badly enough can cause the transistor to fail in a shorted condition. When it does, it doesn't matter what button you push on the CCU, the fan will always run max speed.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]