Don't do it before reading this!!
There's a company in Australia who makes a 'strip type' (flexible) reverse parking sensor that simply tapes inside your plastic rear bumper and works as well as the sensor type ones, it's a strip of what looks like double sided tape with two wires coming out one side that fits internally behind the rear bumper
I saw a CD movie of how it works and how to install it and it looks great! I am even holding off installing my conventional circle reverse sensors because with this one you don't have to drill the bumper.
upside is that it's invisible
downside is that it's invisible! (no one knows u got it)
but, you can buy cheap rev park sensor kits for $20 these days so...
will post web address asap