I also replaced my pads with Porterfields on the ML55. Putting the sensor wire into the rear pads is no problem just have to drill hole into the brake material for the sensor tip, the slot for the sensor head works fine. The front pads are a different story, the slot for the sensor head is in the middle of the pad right under the retaining clip. Were you able to get the sensors for the front pads installed? If the brake sensor light is going on you have a short to ground somewhere, try pulling out each sensor wire one at a time until the light goes out.