Be sure that you don't have a separate blower fuse outside and forward of the regular box. Sort of like a glow fuse. If all this is fine, there is a three wire connector on the firewall next to the brake booster. I think its a yellow wire (but check on your diagram) that is the blower control signal from the pushbutton controller. This signal should vary from 1v to 6v as the blower speed is increased. The blower is under the hood and the regulator is under it. The regulators on the 124 body have huge heat sinks and go bad infrequently. It is a big power transistor that amplifies the output in proportion to the input signal - It is on the ground leg of the circuit)
The blower is more often the problem. The signal from the PBC is not often the problem.
Bosch Master, ASE Master, L1
33 years MB technician