I had the same problem this past week end. Got stuck in traffic and the car overheated. Electric auxiliary fan was not working.
On mine, there were a couple of things that may have caused the problem. One was that two of the three spokes that run from the motor to the rim of the fan guard were broken where the spokes meet the rim. The fixture is made of aluminum.
To solve the problem I epoxied the two broken spokes and the frame now seems to be holding up. I used JB Quick and then put on a second coat of JB Weld.
Also I cleaned the contacts of the plug (connector) that is located inside the motor compartment at the base and to the right of the radiator... near the edge of the headlight. (looking at the car from the front)
I also cleaned the connectors of the temperature sensor and sending unit which is located just to the left of the air breather housing.
My guess is that either the housing spokes are broken as a consequence of fatigue cracks or the fan guard is somehow not attached properly and consequently the fan is being blocked from turning.
I doubt if the problem is with the motor and I suppose the problem is fixable if you are handy enough.
1993 190E 2.3
2000 Toyota 4x4 Tundra