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If you have an android phone, buy a cheap bluetooth obd reader amazon or ebay. They go for about 10-15 bucks

Pair it and use this app to turn off the reverse beep.
https://play.google.com/store/apps/details?id=uk.co.sensiblesolutions.priustweak&hl=en

Very easy to use and you can also shut off seat belt beeps too.

If the constant beeping annoys you, you can set it to a single beep when it goes into reverse. Buy a bluetooth OBD2 adapter and download the Prius Tweak app. There's a setting to change the beep in there.

Found another app that will do less elements than Carista but for free https://play.google.com/store/apps/details?id=uk.co.sensiblesolutions.priustweak