How many blades does your main engine fan have? I think Mercedes changed from a 6 bladed fan to a 9 bladed fan to help with cooling on these motors. I do not know what year that happened though. Your fan clutch should lock or engage when the engine reaches a certain temp. It is usually written on the clutch itself. You can actually hear when the clutch is engaged because your main engine fan will take on a different sound as it engages.
When you engine is cool, you can easily spin the blades of the fan because it is basically free-wheeling. Turn on the car when the motor is cool and let it idle. Take a rolled up newspaper and place it into the fan. You should be able to actually stop the fan. When the engine is at operating temp and you try to do this, you will not be able to do this because the clutch has engaged and the fan is locked to the clutch . Be very careful doing this, you do not want to lose a finger or a hand.

As for the aux fan, it comes on with the A/C when the pressure switch in your drier reaches a certain pressure, not sure what pressure it is with your car. But there is also a fuse for this fan, make sure it is not blown. Does the fan come on when you jumper the fan? Perhaps the fan motor itself is burned out.

Good Luck
John