I linked an on-line version of the evil servo system manual a few weeks ago. In looking at the manual, it states that the switch which detects coolant temperature and permits blower to blow on non-defrost settings only above 125 degrees is on the bottom of the servo. When I ground the sensor in the thermostat housing on my 79SD it turns on the auxilliary radiator fan.
There is also a main vacuum switch which only allows the fan to blow if a minimum level of vacuum is reached. This switch is the left one of the two behind the glove box on my 79SD. You could try jumping that switch.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13