Here is a technial service bulition addressing your complaint. Two most common reasons for this to be happening are metalic window tint and a bad radio the latter of which can be replaced by a used unit from a wrecking yard. The bulliton is as follows. Good luck Steve. please click on the green accept button if you feel as if I answered your question.
APPLICABILITY: All Models
SUBJECT: Radio Static When Rear Window Defogger Is On
If you encounter a customer complaint of radio static when the radio and rear window defogger are on at the same time, there is the possibility of a break in one or more of the defogger grids. When the grid is broken, a high resistance field is created at the break point which can create static on some radio stations. The level of resistance is usually high enough to create a field of interference which projects out from the glass. The break can be detected visually or by using a portable radio.
Permatex(R) Quick Grid(TM) Rear Window Defogger Repair Kit, part If 15067, used for repairing damaged rear window defogger grids, is available at local automotive parts stores
Note: There are certain factors that could lead to unsuccessful testing. Examples are: overhead fluorescent lights, telegraph poles, high tension lines, weather conditions, etc. Testing should be performed outdoors on a clear day. Also, there is the possibility that the static may not be duplicated due to the location of the vehicle. If that is the case, you may want to move the vehicle to a different location when testing. We highly discourage the use of accessory window tinting. If the vehicle is equipped with it, do not continue with the procedure.