On the 124 the blower is rather hard to get to for circuit testing being under the wipers in the engine compartment. There is lots of diagnostics in the archives. Basically you must verify the fuse which may have been relocated outside the fuse box. In the center of the dash is a three prong connector which allows easy access to the next step. Without a wiring diagram I forget the exact testing except to say that the transistor blower regulator controls the current through the negative leg from the motor in relationship to a voltage signal from the pushbutton controller (the regulator is under the blower). The signal wire is one of the three wires easily accessible. It should be a 1v to 6v signal with high blow being the 6v signal.
Bosch Master, ASE Master, L1
33 years MB technician