You dont say where you are, but I had the same problem with my UK A6 unit
The A6 has the aerial built into the rear screen. There is a small box of
electronics that go with it under the rear parcel shelf (on a 4 door) I had
that replaced under warrenty.
It went again (this is a 1999 A6) and in the end I fitted a Bosch helical
aerial and reception has been great ever since!