Have you checked the fan itself by applying 12 volts directly to it?
Is there a separate fan relay? I have one on my 123. When my fan went flakey I merely jumpered it so it runs when the key is turned on. Other owners wire their aux fans into the a/c compressor clutch circuit so it comes onb when the a/c kicks in.
There may two switches on your receiver-dryer. One with a pigtail (on the 123) controls the fan. If the fan starts when you jump those connections, look into the a/c system. Some mechanics replace the pressure-operated fan switch with a temperature operated switch.
But jumpering is a small sacrifice to avoid an overheating problem. On a long trip it has saved me much grief.
If your model has a Kilma board, see other posts on this site for specific Klima suggestions.
Stage Wizards Lighting and sound designer
1981 SD 328,000
1985 300TD, 266,000
1984 300 TD, 267,000