Symptoms: Long tone when reverse selected & sensors inop.
Possible cure: What I found with my car & subsequently people have found after taking my advice is corroded joints in the wiring.
If you stick your head under the bumper there should be a grey multi plug check this by unplugging & visually inspecting the terminals for corrosion, failing that follow the wiring towards the bumper & after a couple of inches you should come across a section wrapped in black insulating tape, unwrap it & this should reveal 2 sets of wires wrapped in blue tape, unwrap these & you should find some crimped metal connectors joining each set of wires together, by now these have probably been wet & will have corroded usually to a powdery peppermint green lump, simply chop out the corrosion, bare the wires back & rejoin using heat shrink connectors & retape the harness.
Symptoms: No long tone & sensors inop.
Possible Cure: Check to see if the reverse lights are working, if not then this would tend to suggest the reverse light switch has failed in the gearbox, unplug it & short the 2 terminals in the plug & see if the lights/sensors are working if they are then replace the switch