I've had no problems with FM reception while using the rear window defogger.

Re: an earlier post, I received Sirius for Christmas for my car. While the manual suggested a top mount at the rear, I luckily had the forethought to position it and then open the moonroof. It was way too close for my comfort so I mounted it on the trunk lid. It works great. Later a custom installer told me that he suggests all cars with moonroofs should have their satellite antennas installed on the trunk. With a top mount, even though it is small it adds to the wind noise when mounted on the front.