2006 audi a4 cabriolet 1.8 t reverse sensors clicking on reverse but not working

The sensors in my car are making a ticking noise when put on reverse but they are not working and no sound is coming when coming to reverse contact . Someone said on here that if you here a ticking noise when put on reverse it's that it's trying to connect . In just wondering what this means . Does anyone know if it's a fuse or something and where do I start to get this working .anyones advise appreciated