Your vehicle has a lighting control module and it gets a signal from multiple switches to operate the lights. You have to determine if the turn signal switch is not sending the lighting module the command for left turn signal. If the turn signal switch is ok, then you probably have a bad lighting control module. You will need to get a wiring schematic and a voltmeter to do these tests, if you do not have either I would recommend finding a shop specializing in Mercedes Benz to check it out for you . Good luck.