I have the exact same problem on my recently acquired 2000 C230 Kompressor sport. I've done some troubleshooting/research on the aux fan control scheme used in these cars and it seems that the Climate Control module (N19 or N22) is responsible for generating a signal to activate the auxiliary fans. I've also discovered that if the climate control panel is replaced it must be "version coded" using a special software application that typically resides at, you guessed it, the local Mercedes-Benz dealer. There are several posts in the archives about aux fans not working properly due to incorrect "version coding" of climate control modules. Ask your mechanic if they set and verified the version coding of the climate control module (Module N19 or N22) when it was replaced.

I'm not exactly sure what "version coding" is but I'd guess it is used to set-up control/information parameters, protocols and features that are specific to a particular vehicles hardware and software systems. Kind of like getting the right printer driver for your specific hardware/software environment.