Dead Parking Sensor

I have a dead rear parking sensor on my A6 C7 Avant S-Line. Can anyone confirm if I can just pop the lower sports defuser off and change it from underneath or do I have to remove the entire rear bumper?

Also does anyone have any experience with third party sensors from ebay, any makes to avoid? Big difference in price from them to the OEM parts, I could change all four for the price of a single audi one!

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I got to mine without removing the bumper just jacked rear of the car up mand reached it np
 
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