sounds like you have a faulty sensor and someones swiched them off to stop them making the noise the kits are the same as the kits fitted to toyotas so you can get replacement sensors
to find out which sensor is not working switch them on car in reverse sensors sounding in your case
lightly touch the surface of the sensor with your finger you should feel it click very gently
each sensor should do this if you find one whcih doesnt check the wiring to that sensor has not broken
if thats ok it will be the sensor thats gone down
you can confirm that by switching sensors with another and seeing if the fault moves with the sensor!