Rear Window Defroster W/Happy fuse

I just bought a 2003 Silverado 1500HD w/66000 Miles on it. I love it but for the small quirks I need to fix. The main one I'm going to concetrate on right now is the Rear Window Defroster. It Keeps BLOWING the 30 Amp fuse. I put in a new one and the second I hit the defroster button you can hear it blow. you ca also hear the relay under the dash. I tried to disconnect the window grid but it still blows the fuses. The heated mirrors work fine though, except for a buzzing sound that it makes at speed. that part is not the windshield seal like the dealership said it was. It stops the buzzing when I fold in the mirrors. any Ideas would be GREAT. thanks for the time.