I just repaired the rheostat in my 300D w123. Major problem was that the rotary slider was pushing the resistance coil out of its groove. Second problem was a high resistance connection between the end of the resistance coil and the rivet.
The free end of the resistance coil was fixed in its location by a small dab of muffler repair putty (silicate formulation) at its tip within the ceramic base then heated in my wife's oven at 150 degC for 30 minutes.
There was a grey powder like deposit at this point which may indicate a similar compound had been used previously -obviously it would need to be heat resistant, because the dimming function would be dependent on shedding excessive watts as heat. The high resistance connection was fixed by using a square ended awl pressed into the rivet to oscillate it backwards and forwards (about 45 deg total). The rivet was then firmed up with a punch very cautiously (to avoid cracking the ceramic) while backed up with a second punch in a vice. Resistance of the connection was then checked at 1 ohm or less.
I tied the rotary slider to its post with a few loops of cotton thread to help in replacing it. They were easily cut and pulled away when the assembly was complete. It is important to bend the retaining arms at the top plate with using needle nose pliers to obtain a firm re assembly.
I lubricated the sliding surfaces with dielectric grease after reassembly. Otherwise everything is too sticky and slippy.
While an ohmmeter indicates regulation over about 10 ohms, time will tell whether all this works.
Andrew