Let me dust off the cobwebs a bit. I can't remember which fuse it was, but those cars had separate fuse boxes for all the options.
Too many cobwebs, had too get the book. The relay for aux fan should be one of the round relays near the EFI ECU inboard of right headlight. There is another relay (round on the firewall) that is called the separation relay (it separates the activities of aux fan and timing control). The fuse box is a single two fuse panel on the firewall. In the picture it sits at an angle.
The fans on the 108/109 bodies also had a hood switch to keep vital body parts from suddenly becoming engaged in a fan coming on. This switch (a white button) must be depressed for the fan to come on.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician