Owners repeatedly flag the heater system as the platform's most dreaded job. The heater blower is buried behind the dash and replacing it is described as a month of weekends and a two-thousand-dollar hit to value; the plastic heater/ventilation levers are known to snap off within years, leaving owners fishing behind the dash to work the controls.
Heater blower and heater levers: costly, notorious failure points
Mercedes-Benz 250S-300SEL 6.3 (W108/W109) · mechanical