Any ideas in which direction to look other that l/r dealer plug in (bend me over a barrel and lots of lub first haha).Driving home the other night no problem reversed into drive and got the 3 second tone and flashing switch,now its all the time.The sound is from unit in the boot can't hear any clicks from sensor but it would appear system shut down although unlike last time when front sensor went no message up on dash screen, odd that .It had 4 new front sensor this year ??.I don't this thing likes crimbo time cost me £800 on EGR valves this time last year.BAR HUMBUG.

just to add i've just been out in car and the 3 sec sound and flashing switch light happen as soon as ignition is turned on to position 2 even in park

Wiring loom in front bumper did exactly this on mine.
Dont think its an expensive or difficult fix

ok mate will check that in morning looks like bumper off !! again

Finally got time to do checks,i'm getting two fault codes one saying no supply two front sensor circuit and the other the same but to rear, checked both plugs and 12 volt at each if unpluged from bumper looms but drops to 0.4 volts when connected
most odd ! could this problem lie in the ecu module.Any thoughts the codes are B1B58 Rear sensor circuit power loss.and B1B57 front sensor circuit power loss

update 2:- The problem is instant as soon as i put in ign key and turn to position 2 ign on and not running the 3 second beep and flashing light start ,now i see most posts are saying on selecting reverse this happens but not in my case and as i said fault codes are saying no power to front or rear sensor circuit so they will not tick will they as not powered up could this be the control module has died is it common for them to go .most electrically un-relible i have ever owned any help to restore my faith most welcolme

update 3:- having spent all day and both bumpers off doing all the multimeter checks going,cleaning out crap and sealing plugs i think i'm nearly there system is now sort of trying in 3 sec beep gone but no sounders re coupled my icarsoft reader and did a live data check and now have supply back but 1 rear sensor has died and one front (new but cheap import crap from guess where folks mistake) has gone already salt on roads i suspecting but who at R/R designed something electrical to be located in direct firing line of the rear wheels .SPOT ON that one.watch this space

Problems all sorted for future reference DO NOT buy cheap sensors that come from china although they are listed in the UK

Thanks Vam17Hel

Duly noted! Glad you got it sorted!  2015 RRS AB SDV6, Fuji White, Ivory
2012 RRS HSE SDV6, Sumatra Black and Ivory
Audi A5, 3.0 TDI, Gone
Audi TT, 3.2 V6, Gone
BMW 5 series tourer, Gone