Sorry I dont have an answer, but dude, how often do you use your defogger? I've used mine like 2 or 3 times a year. Actually I mainly use my a/c to defog.
There is a difference between the defroster and the defogger. The latter is for the front windshield which can only be operated with the AC or heater and the former is for the rear window. I experience the same thing when I turn my defroster on. I presume it is EM interference since the radio antenna is inches away from the defroster.