Does a sensor light come on when the brake pads need to be replaced? on 2011 Mercedes-Benz GL450

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My 20,000 mi. service check light just came on. My tire guy told me that I needed new back brakes and that the rotors always have to be replaced with the pads on my car. I just took the car to Mercedes for the check light and they tell me that my front brake pads need to be replaced, but not the rotors. Who should I believe?
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