got this little thing through ebay for a very good price, now i gathered some infos about it:

when putting in the rear gear, the LEDs show you the distance to some object you're backing into (well,hopefully not ^^) and some piezzo device will give you a corresponding sound...

Complete set (fits all 3rd gens!) : BL4C V7 290A (~360 euros retail price)

replacement parts:
1 piece(s) replacement sensor: 4100 78 656 (65 euros)
4 piece(s) sensor installation housing: 4100 78 655A (17 euros)
4 piece(s) contour rings: 4100 78 650 (4 euros)

picture attached

 

Sweet, not that I really need this .. but it's a good mod to do that should increase the selling price of your vehicle later on. Kudos to you my man