I've installed the CA-5004 from Crimestopper in my Prius. Works great!

http://www.precisionautosound.net/crimestopper.html

scroll down aways to see the backup sensors.

Let me know if you have any more questions. It was easy enough install, but I'm an installer of car gadgets in a former life...you do have to drill holes in the bumper area.

I have pics if u need them, they are on my home computer, and I'm at work now.

'04 Seaside Pearl #7. Fumoto oil drain, mudflaps, rear bumper scuff protector & rear warn system, compass mirror, EV mode button, 8" subwoofer in right rear cubby & 6" subs under seats, power lumbar in the front seats, Coastaletech hitch w/ Aspen bike/snowboard rack. iPod2car, 2 amps, Alpine component speakers, and DVD video, solid 47 MPG @ 70000 miles.