From PriusChat.
# How do I turn off that annoying beeping in reverse?
1. Power on (not necessarily ready, IG-ON will do).
2. Set Trip/ODO to ODO
3. Power off.
4. Power on to ready (brake on).
5. Within 6 seconds, press and hold ODO for 10 seconds or more.
6. WHILE STILL HOLDING ODO *after* 10 seconds, Shift from P to R, then back to P. Now release ODO.
7. You'll see in the trip display "b on" or "b off".
8. Press ODO to toggle.
9. Now power off.
(If you disconnect the 12v battery, you may lose this change.)
You may also disable the seatbelt beep, but I do not recommend it.
As always, performing any modification is at your own risk.
Cheers, Doug