I believe your car has the M104 engine.
You can test the operation of the fan by un-pluging the ECT sensor harness. Some say the aux fans should turn automatically. For my car, W140, this test dose not activate the fans. But I have read many W124 turns this way.
If it dosen't turn, than you can use a resistor and connect it between the harness. Basically the fans come on at different speed depending on the Ohm reading from the ECT sensor. By puting a resistor between the harness it will trick the computer thinking the car is hot enough to turn on the fans.
How many ohms??? I don't have the figures with me but if I remeber correctly, 300 ohm should do the trick. If not, I am sure anything lower like 2 hundred something will definitely do the trick. Let me know if you need exact figure. This test also help you to test different stage as the fans at different speed depending on ohm readings