The parktronic is a reverse-sensor that senses any objects while reversing. It lets out a tiny moderate beep to tell you as is well ... just go on reversing.
The beep gets louder as it detects a presence of a solid object - sometimes lamp-posts will be detected, other times, just rely on ole' conventional methods that go bup.
Its useful when reversing into tight spaces, and when the beeper yells BBBBBEEEEEEEPPPPP!!!!!!
*Stop*. Thats enough.
Its not that hard hooking it up on the S-class as the installation kit comes with a manual thats easily read and understood. You will know your rights for sure.
OEM would cost about 3 times more than the not-so-original part.
ps. and I forgot to mention that sometimes to sensor works overtime when it rained heavy, or the sensors are covered with some loose dirt or leaves ...