Aftermarket Reversing Sensors - Cont Beeps

[Gatt]

Hi there,

I have a 55-plate Passat 1.9 TDi S Saloon and trying to install some aftermarket reversing sensors

It's one that goes behind the number plate.
I've been through and installed it and wired it into the reverse light (after a lot of googling for how to do it!)

However, when I put the car into reverse, it continuously beeps, despite there being no obstruction behind me.

I've checked that the sensors are clean and clear of dirt which, to me, is the most obvious cause. But its still beeping.

I'm now confident that the sensors are wired correctly (after many attempts!) as the sensor is making a noise.

Has anyone else had experience with these type of sensors and if there is anything I've missed?