Hi offnote
When I got my van it had four retrofit sensors in the back bumper. It never worked well as the sensors could "see" too much of the road behind and gave a lot of false alarms.
I got some little angled wedges from Dolphin to add between the sensors and the bumper, which makes them point upwards very slightly. After that it worked a lot better.
These rings ideally slip over the wiring during installation, bat as I didn't want to take it all apart I carefully cracked each ring, slipped it over the cable and then superglued it back together !