Other than a defective motor, a bad switch or blower motor resistor assembly can cause an inoperative heater blower motor.Google+
Car Problem Reports
Mercedes-Benz 560SEL Bad Switch/Blower Motor Resistor Assembly May Cause Inoperative Heater Blower Motor
Mercedes-Benz 560SEL Problem
Average Mileage: 149,333 mi (130,000 mi - 188,000 mi)
O MY GOSH, 1990 560 SEL: Blower motor, Heater motor, I checked fuse #5, and the fuse in the black casing (Strip fuse) next to fuse box, ALL FUSES ARE GREAT, Heater motor wont work. I checked the Heater blower motor behind the glove box, FOUND OUT THE GROUND (Blue) Wire is bad! I connect a wire from ground wire to metal somewhere else on the car, and The blower works! BUT! IT won't shut off, it will not turn off once I press the switch on the center console. It only works on high speed now! And the "OFF" switch will not work! Helpppp!
AC/Heater blower inoperative....trouble shooting.
HVAC blower motor shorts out / cuts off regardless of speed setting.