Good point. The switch is the red or green switch that has two wires coming from it. Just bridge them together with the key on and the aux fan should run. If so you just tested it and it is fine. If not the fuse in the box (c or d) is likely blown. If the fuse is OK it may be the pre resistor.

------------------
Benzmac:
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM