Why do I hear static in my stereo when the rear defrost is on? When I turn it off there isnt as much!?

I have a 2003 Honda Civic LX 2dr and I had a Pioneer stereo installed back at the beginning of the year. As you know, these Civics do not have an external antenna. I think they are built into the rear window. I do have black tint on the back as well. When I turn on the rear defroster, I get static and when I turn it off it goes away. I also have less static a lot when the heater isn't on. The stereo was installed by circuit city so I am sure they installed it correctly. There wasn't any problems with it after they installed it until now. I read somewhere it could be my coil in my defroster. All I know is, there is static and what do I need to do to eliminate it? Is there antenna I can by to boost the signal. Any input would be greatly appreciated. Thanks