I had the same
problem, if you check this on any audi forum you will see no shortage of replies... they used the same system in the b7's. the most common fault seems to be the antenna circuit board cracking with the extreme temperature differences. follow tonyuk's instructions above, its located on the top right hand corner behind the plastic trim along the top of the window.
1) get your fingers up between the rear screen and the plastic trim, give it a pull and you'll hear the clips popping.
2) You will see the antenna on the top right hand corner, it has three cables in one end which have colour coded plugs. unclip the antenna, cables and earth clip.
3) check the black rubbery/plastic coating on circuit board for cracks, if you see any its likely they have damaged the circuitry underneath. new antenna required.
4) if there's no cracks try cleaning the sprung connectors and the terminals where they make contact with the heating element in the screen with fine sand paper (which was the issue with mine)
I had my radio on when I was doing mine so I could tell if/when reception improved.
Failing that, your next job will be checking the cables.....