huh....i've had my rear defrost on with no probs to the radio.

my best guess is that there is either some crazy feedback that the defroster is causing, which would mean that there may be some connections in the lines to or on the window. you could look over the window and look to see if there are some irregularities in the the way the defroster and radio are run along the window and that nothing is bridging the two together.

thats the best i can suggest