Other than a defective motor, a bad switch or blower motor resistor assembly can cause an inoperative heater blower motor.
Car Problem Reports
Mercedes-Benz 420SEL Bad Switch/Blower Motor Resistor Assembly May Cause Inoperative Heater Blower Motor
Mercedes-Benz 420SEL Problem
Average Mileage: 179,100 mi (145,000 mi - 230,000 mi)
Visitor, 1990 Mercedes-Benz 420SEL, 155,000 mi
when I place in key and start to turn car on blower will come on sometime when I start the car blower goes off most of the time the blower does not come on at all.
the blower motor was rewplaced 2 years ago and it quit working but i hear noise when i push buttons on air condition compressor is fine no air comes out of the vents
Visitor, 1991 Mercedes-Benz 420SEL, 175,500 mi
you can hear it turn on but no heat or a/c come out of my vents