Hi 190E2,
I had the similar problem you have on my 88'300E.
But for your problem there are two posibilities:
1. The thermoswitch, which is located on the cylinder head. At that time if I remove the socket to the thermoswitch the fan run at high speed. So my conclusion is thermoswitch is good.
2. The resistor located on top of the front tyre mound inside the trunk. Possibly on the left hand side. (Mine is a right hand drive car). It is a rectangle thing made out of ceramics with two cables on either end. To check try put a jumper between the two cable, if the fan run, then you should change the resistor.

Hope it will helps you.
Bye