If you aren't being blessed with a long tone when you put the X in reverse, the sensor is likely working. You can test them by activating the Pdc system and listening for a clicking sound being emitted from each sensor.

The sensors mount into a "holder" that mounts to the bumper. The extra corner ones I have in a box look like they clip onto the bumper....not use 3m sticky tape like the one you have linked. (Hopefully someone can Confirm this though a I'm not currently at home...)

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