The switch is almost never the problem.
It is usually the relays or fuses. To verify the whether it is the switch, unplug the two wires going to it and join them. If the fan works either the switch is bad or the A/C has too low of a pressure to activate it (another likely senario). So the next step would be to put a gauge on the high side and verify the pressures.
I just looked to see what the pressure was (20bar = 284psi) and looking at the wiring I remembered what the most common problem was: the ballast resistor that makes the low speed. It burns out like a big fuse.
Bosch Master, ASE Master, L1
33 years MB technician