Viermalvier.de, das Geländewagenportal Foren Marken talk Mercedes Rückfahrsensorik 463

(Leider auf English)
No need to spend so much money on such a simple technology.
The used bumper I bought had the holders for the sensors. For $13, I bought a cheap-Chinese backup sensor kit (ex:https://www.ebay.de/itm/4-Sensoren-Parksensor-Ruckfahrwarner-Silber-Ruckfahrwarner-Einparkhilfe-PDC/362939019313) and glued the sensors into the MB holders. Works perfectly, no visible difference.