If you are missing any of the sensors (Front or rear), then the park assist module will throw fault codes and will not work.
It is apparently common for the sensors to fail, (and the problem is only a Tech2 tool can read the fault codes for these things to find out for sure).
So it may have been a case of the previous owner having a problem with one sensor, and thinking stuff it, the park assist doesnt work, so there is no point fitting sensors to the new bar.
Mine hasnt worked for almost 2 years, but I just got it scanned last weekend and found the front right middle sensor is open circuit.
(Hence the park assist module not working for me).
I hope this helps ?