I am struggling with this fault also-you'r not the only one! I get a continuous tone when I select reverse and have therefore pulled the fuse on the sensor circuit, however when I select reverse with ign. position 2 I cannot hear any clicking from the sensors and assume that the fault is further back.The parking aid module is located on the LH side of the spare wheel bay.(I only have rear PDC)
I have studied the wireig diag. and am familiar with the circuitry and when I get my car( it's on the high seas between UK and Oz) I think I will remove the module and look at the pcb and components using a magnifing glass and look for signs of burning, shorting or bad connection. The other thing one can do is look for signs of corrosion on contacts. Good luck.