I have an Audiovox ROSS [rear object sensing system] in my 300M and have found it to be too sensitive to be a lot of help to the driver. It has a flashing light on the upper corner of the rear window [yellow/red] for visual aid and two small boxes mount under the bumper. It beeps in certain patterns as objects get closer to the car. The beep is only heard in the car and I have it turned up to max [drives my wife crazy] so I can hear it over the A/C and stereo when I start the car up. Both come up loud, especially in the hot summer. My installer has tried shortening and narrowing the unit's detection areas, but to no avail. It picks up things near the side of the back end as well as straight back. Thus it is hard to tell if the ornamental bushes are causing it to beep incessantly, or if I am about to run over something behind me. Cost was about $300 installed. Can't recommend that brand. The Direct Electronics unit mentioned above sounds like it is more precise.