It may be that the motor itself is bad. That was what I encountered on mine. I removed the bottom cover under the motor/resistor-regulator. FUPABox on YouTube told me how to check the motor for bad brushes: To turn on the ignition and blower switch in positions such that you would normally cause the fan to distribute heat or A/C. If the blower does not run, locate the bottom of the motor shaft at the center of the bottom of the blower motor and push up on the shaft. If the blower then runs, you probably have worn brushes in the motor and need to replace the motor, for I do not know of a source for the correct beveled brushes for this motor.
I transferred the old resistor-regulator to my new motor and installed the motor with blower cage using the Torx screws and it works well. I have a SLK320 but the bodies and engines are similar.