Be aware that MBs have softer brake pad linings than most cars that I have owned and wear a good bit quicker. Maybe it's just a heavier car and a lead foot. In any event, the pads should have wires that are inserted into the brake pad material. Once the pads have worn down to that wire, it makes contact with the rotor and closes the circuit for the indicator lamp in the dash. It will start out by pulsing and then could go away for a while and reappear.