I have a RNS 315 in a 2011 Sharan. Recently I changed the battery.. After that I have a blank screen most of the time. The radio comes on and off randomly at a low volume. None of the buttons work. The clock goes on and off, Climate control only shows sometimes on the screen. The touch screen will not respond when it is on.. I cannot use the steering wheel controls for the unit It has been to the garage and is showing no faults. All the fuses are fine. any help would be apreciated.