Had this problem on our '53 Alhambra, cured it by bending in the plate holding the tow plug as it was interferring with the sound path on the two middle rear sensors. All work fine until a front sensor failed, unfortunalty my VAGCOM can find the controller for the sensors, which is really annoying as I tried to swap the non working sensor with a replacement one and the fault stll exsists. Maybe the wiring behing has been damaged as suggested in an earlier post.