Fitted new reversing sensors.
A bit annoyed as I had the rear bumper resprayed last year, re-assembled everything and the sensors worked fine. Then during the winter I had the dreaded constant siren as soon as reverse was selected. The sensors are original factory fit, so bought a secondhand control unit as that was the easiest thing to try and eliminate. Just the same.
Ordered four new sensors from Skandix, removed bumper and sensors, sprayed up new ones and all re-fitted. I tried to carefully brush paint the sensors as I had some paint from the re-spray. It looked cr*p, so stripped the paint and started again. Flatted with 1200grit, panel wipe, two thin coats of primer, two dust coats of colour - aerosol colour matched paint - followed by two coats of clear; then 12 hours in the airing cupboard to harden. Almost perfect finish
Two things to note: the Scandix sensors are exactly the same as the originals and I was pleasantly surprised how easy the old sensors come out of the bumper once that is removed and stripped - they just unclip from their holders.