B5 crummy reception w/stock head

1999 B5 V6 w/stock VW AM/FM/Cass head unit. Radio worked great for first three years. Now in the past three weeks the AM reception has gone out completely and the FM reception is very, very weak. Any ideas on what to check? Antenna looks good and no corrosion on the Fuba threads. Unit has never been out of the slot, so I haven't checked for loose connections on the DIN. Anyone else have this problem?
I have the MasterGuard Platinum warranty, so I will get it fixed free no matter what, but it would be nice to see what's wrong first.
Drive it like ya' stole it,
Mike