[FloridaMerc]

Have you had it on SDS yet, and if yes what codes were present? Not much sense in arguing about it until we have those. With that message on the IC there will almost certainly be codes present in SDS.

Code P0694 - The output of electric fan has a short circuit to ground.

I went ahead and tested the cable wires that connect to the radiator's fan, the Power (thick red), the ignition power (black striped), and the Ground all working. I don't have an oscilloscope to test the PWM wire.
I could try replacing the Relay "I" which is supposed to be for the fan.

I hope it is not the ECM. The fan does not always fail, but sometimes (sometimes the car goes longer periods of time without throwing the error sometimes less) it does by trying to engage (moves just a little bit) then it stops and throws the error.

 

[FloridaMerc]

for the SLK350 R171 does the radiator fan have a built-in module to control the speed or does it all comes straight from ECU?

 

[efair]

There is something digital in the fan, IDK if it's a "module" though. I don't have any experience with it other than removing and reinstalling my fan.

My guess here would be your fan needs replacement.