For instance you can try this:
or this: http://www.maplin.co.uk/4-sensor-wir...ing-kit-265097
or one of these:
(not sure about the wireless bit!)
And I'm sure others are available.
It was pretty easy on the scenic, but I ended up having to remove the bumper outer just to get the holes done, so you may want to plan to do that in any case.
Basically - 2 (or 4) holes are drilled in the bumper, feed the wires through and into the car, this tends to be in the area of the rear lights, so you can connect to the reversing light circuit.
The sensors are supposed to be rotated so that one side is up, but I'm not sure how much it matters.
The module is located behind the rear lights normally, but whatever suits best - it doesn't really matter where it is as long as there is an earth connection and a reversing light.
Once the module is installed it's then a matter of running the display to a suitable location, normally the dashboard or, as I have it, on top of the reversing mirror.
In operation it's pretty simple - once reverse is selected the unit is energised and when any of the sensors detects something it will beep and the display will lightup with bars and a measure of how far away the object is.
As you might expect the nearer you get to something the faster the beeps, so you get a good idea of distance.
For the money and a few hours work it can be very useful.