Above brake pedal or integrated in the master cylinder. It depends on the year.
Check above the brake pedal first. If it is not there, then it may be part of the master brake booster. Since I dont know the vin and details of your car, I can't tell you for sure. If you can't find it call your a Mercedes-Benz dealer in your area and provide them with the vin number. They can look it up for you.