I had a similar problem on a 1991 300e 4matic in the UK, 205,000 miles

When I bought the car it would overheat quickly, and max out when stationary. A new thermostat (very cheap and quick to fit) cured that problem immediately.

During the diagnosis of this I noticed that neither of my 2 electric auxillary fans worked. I managed to get one slowly back into life by tapping the centre spindle lightly with a small hammer. Following this I drilled a small hole into the plastic cowl which covers the centre of the fan motor and filled this with WD40 spray lubricant using the straw that comes with it. Shorting out the temp sender I let the fan spin for 5 mins to free it self up. You may wish to silicone up the small hole once all is well.

However this method hasn't worked on both fans, but is worth a try as a temporary fix. I think the power lead may have become detatched from the other, and it's not easy to get access to.

In any event with the new thermostat the car doesn't get hot enough to turn the fans on (105 degrees I think) and my aircon is out of gas which is the other trigger for turning them on.