Well the basics are pretty simple. There is a three wire connection on the firewall that tells most of the story. The story should be that the PBC (pushbutton controller) sends a 1v to 6v signal to the blower regulator. Both this signal and the power pass through that easy to reach connector.
This simple point can diagnose whether the problem is blower regulator/blower motor, power supply, or control signal. Get a wiring diagram and find that connector and make this determination. Depending which area is the problem will determine the next step.
There is loads of info in the archives about this. There may even be a picture of the connector although that isn't hard. The wiring diagram as always is a must.
Bosch Master, ASE Master, L1
33 years MB technician