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 SL500 Bad Switch/Blower Motor Resistor Assembly May Cause Inoperative Heater Blower Motor
Mercedes-Benz SL500 Problem
Visitor, 1998 Mercedes-Benz SL500, 52,000 mi
OK when purchased in March, 2013. One or two instances of blower malfunction (slow speed). Seems to be working at present (Aug 2013).