Reverse Parking Sensors
These parking sensors can be easily fitted to your car in little more than an hour or two, with just a two-wire connection to your reversing light bulb which switches the sensors on when you select reverse gear.
Most systems use ultrasonic technology; emitting a sound wave behind the car which bounces off other objects and returns back to the sensor.This calculates the distance between the object behind and your car, based on the time taken for the sound wave to travel out and back to the car.
A few systems work on an alternative basis; generating an electromagnetic field which measures the disturbance caused by obstacles within that field as you reverse and letting you know when you're too close.