As far as I know, it needs the A/C pressure switch to be active.
Are you sure your A/C is working at full potential?
The pressure switch should be found somewhere near the A/C compressor on the high pressure line (I belive).
I don't have the diagram in my head any-more, must have misplaced it up there, but believe there is a relay in between. So, pressure switch activates relay, relay closes the circuit to the fan.
Have you had a look in your relay box and checked the fuses too. The fuses should be marked as such inside the lid unless lost.

I found this, maybe this is what you need Pelican Technical Article: Mercedes Benz - Auxiliary Cooling Fan Belt Replacement

__________________
Mostly, I don't know notin, I just know where to look.
I am looking back, to over 30 years in Electronics Design.
Electrons don't care if they move in a car, computer or relay!

95 W124 E320 M104.992 - Because, I love to repair, naaaah!
Over 221,000 Miles
Cheers,
Norbert