Where is the Ariel on the car? I have had problems with my radio in my car..
I have an E36 BMW, they have the Ariel in the rear window rather than
externally. These cars also have a booster actually as part of the car, not
the stereo, what can happen is this booster stops working or the connections
become loose over time and this causes poor signal, and in extreme cases it
loses it all together.

First think I'd do is obviously check the connections at the back, and see
if you can check another friends radio on your ariel. Also might be a good
idea to have a look around and see if the passat has this booster, I'm not
sure if it's BMW specific.