Ultrasonic devices are used with some good results. Found a 12v one designed for underhood use on Ebay for about five bucks. Bought two - one for toad and one for coach. Will let y'all know in a couple years if it does not work.
Suzie & Chuck USNR USAF USCGA Challenger Owners Club 2015 37KT Challenger, Demco Commander tow bar, Ready Brake, Sewer Solutions, Safe-T-Plus 41-230, 5-Star tune with 2014 Ford Fiesta behind us & 385 FastTrack kayak inside.
Summer - Creede, CO Winter - Okeechobee, FL