Take a look in the Do It Yourself section of this site and read the Brake Job article. You will see the brake pad sensors there. You may have sensors on the front and rear pads. Without knowing what car you have it's impossible to tell. The light comes on when a pad wears down to the sensor and the sensor is grounded to complete the electrical circuit.
Bottom line, you need new brake pads and sensors. You can visually look at them to see how far they are worn. They come in sets...front set and rear set. Use only Mercedes OE quality pads from companies like Pagid, Jurid, Textar. We sell them in PartsShop. Go to PartsShop FastLane and enter the year and model of your car to see what we have.

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Bill Wood - Webmaster
MercedesShop.com, LLC