On top of that, the aero package is also another case where the bumper will have to come off in order to replace one of the sensors. As about finding the dead sensor, it can be done either through a diagnosis tester or by touching/listening to them, like the thread flyingmachine mentioned. Other culprits can be bad wiring, loose connectors, water infiltration creating a short, or even dead PDC module (the one in the trunk).