Could be the rear Object Sensor itself, or the Parking Assist Control Module, located under the passenger seat. Replacing the rear object sensor requires removal of the rear bumper fascia and re-painting the face of the new sensor.
Replacing the parking assist module requires removal of the passenger seat and re-programming of the new module.
Maybe you could check the wiring on the bumper sensor by scooching under the car and having a look.
There is also the buzzer (alarm) itself, which is located in the rear right side of the tailgate garnish molding.
So it could be that the sensor is working but the alarm is not sounding.