Hi Synbrid05
Suggestion: In your profile, indicate the model year of your Prius. Might help a lot on responses since model changes could affect procedures.
In my 2004, here's how I disabled the "Backup Beeping"
1. Power ON;
2. Set Trip/Odometer to "Odometer" (If you're already in Odometer mode, you need to exit and re-enter Odometer mode);
3. Power OFF;
4. Power ON (foot on brake) be sure "Ready" shows;
5. Within 6 seconds, press and hold the Trip/Odometer button for 10 seconds or more - continue to hold it down;
6. After the 10 seconds, shift from Park (button) to Reverse, then back to Park, then release the Trip/Odometer button
7. The Odometer should read "b on" (where you would usually see the total miles);
8. Toggle to "b off" with the Trip/Odometer button. When the "b off" shows, your backup beeper is disabled.
NOTE: If your 12v auxiliary battery is disconnected, the system will default back to backup beeper ON.
Can't attest this works on the 2005 and 2006 models but worked on my 2004. It was just a little tricky.
GOOD LUCK