Remove the cover at the back of the engine comparment, pass side, in front of the windshield. There are about six big phillips screws holding it. The blower is underneath. On top of the blower is the regulator. It controls the current flow (speed) of the motor in response to the voltage signal sent by the pushbutton controller. This signal should be a 1 to 6 volt signal. They may go as high as eight volts but the one I tested last week wsa getting a little over 6v at max.
If your control voltage is right then the regulator is probably bad. (That is the very common answer to your problem). Another common problem is a air filter that is plugged.
Bosch Master, ASE Master, L1
33 years MB technician