Hi there,
I believe that the switch to turn on the fan is actually a temperature sensing switch, at least that's what it is on my '82 300D - it senses whether the temperature of the coolant coming out of the condenser has gotten hot, and if it has, it turns on the fan to cool it. On my '82, the fan usually only comes on in slow traffic or if the vehicle is stopped. In very hot areas, it might be a good idea to switch on the fan all the time, just to make sure there's less load on the compressor.
Regards,
Richard Wooldridge
'82 300D/4.3L V6
Etc...