Good4Sumthin

Hi Can anyone point me in the right direction please I have a 2005 SC430 and can't for the life of me work out why the rear Parking sensor stops working from ime to time. I don't have a cut off switch in the boot, The sensors are always clean. And each time I take it into the dealer it works fine (they must think I'm bonkers ! Is there a switch on the dash that I'm flicking or could it be a loose wire (if so where should I look). Kind regards Dave