2001 Mercedes C240 sound system problems

This car has the factory Mercedes AM/FM Cassette stereo with a seperate 6 CD changer in the glove box. It always sounded great until this past winter. The FM has some slight static on every station. Hardly noticeable, but enough to know there's a problem. I thought it was a blown speaker at first, but the CD's still sound great. When I turn on the rear defroster, the static is so bad you can't hear the radio. Any suggestions?