I have a 63 plate a8. I have cameras on my car but if i select reverse all i get is the parking sensors on the screen. If i push the p on the dash same happens. 

I sometimes have the display go goble de goo .. just lines and random colours if i close it and open it sometimes it works  sometimes it goes blank.  

Ive been told its a cable and ive been told its the module ..... have reset the mmi system but not worked

The things you have been told is wrong, was this done via a diagnostic check to see if any fault codes flag up? 

I'd recommend a diagnosis at an Audi specialist. 

It could be the module, cable or the camera