Hi
This is a common problem as many professinals talked here. You may have a burned fuse in the low speed fan relay.
Here is my experience on C220.
Find the relay box under the hood in driver side right next to fuse box. Screw off 5 screws then open the cover. You can see a relay with 15A fuse, this is the one for low speed fan. Replace it with 25A or 30A cause 15A is too conservative for the current the fan consumes.

I also know it from this great site. Thanks to all who contribute to this site.
I am so glad to have my first post here.

Charles
1994 C220
TAIWAN