2002 Mercedes Benz S430 sensors

My breaks were replace (Front and back) as were rotors, they forgot to put new sensors in. Sensor indicator comes on as you start car. My question is how to replace the sensors. Can it be done without taking off the new breaks and rotors? I know cost is about $7 per sensor and I am not sure how many. Having problem getting them to replace with out additional cost. I ofter change breaks etc so I can do some repair myself. Where are the sensor loacated and how difficut to replace. Thanks for any advice.

J