The defroster grid on the rear window is made of conductive paint. If one of the grid lines has been scratched, then the connection has been degraded or disconnected on that section. This causes the static in your radio. You can purchase conductive paint repair kit from GM or other places. After you find the scratch, put masking tape on both sides of the grid line and apply the conductive paint to complete the connection. I've fixed several rear window defrosters like this.