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'98 MB E320 Blower Motor Temperature Control

I own a 1998 E320 Wagaon and have been told it needs a new blower motor (which means a new regulator too). I am skeptical because when the defrost-only button is engaged (when the car is off and the key is in the no. 2 position and sometimes when the car is running) the motor works fine and blows hot air. Without the defrost-only button engaged there i cannot engage the blower at any speed, direction, or temperature. Does anyone have any thoughts what this could be other than the blower motor? Thank you.

It does appear the motor is okay but there seems to be a problem in the HVAC unit that controls the motor. The shop may want to replace the motor due to the age of it. I’m not familiar with the design of the system but it sounds like the defrost cycle bypasses the HVAC system somehow.