The Jaycar Proximity Parking sensor works on an Ultrasonic pair, (One sends an Ultrasonic Frequency, the other recieves it).
Because Ultrasonic Sound (>40Khz) is very directional, any object that comes close to the Sender or reciever will disturb the sound generated, and will give a reasonable idea of the distance the object is.
The Ultrasonic sensors are installed outside the vehicle and are about 1.5cm in Diameter.
You will obviously need to mount these somewhere, and Drill a very small hole to run the cabling for them.
The US pair are weather proof (Doesnt matter if they get wet), and the Amplifier/Reciever Board should be mounted inside the vehicle, so as it doesnt get wet or anything.
Also the Unit runs off 12V.