2004 Chevy Malibu Electrical Short

The interior lights do not work. I have replaced the fuse for the lights and it continues to blow. I have had the vehicle into the dealership twice and they can not diagnose the problem. They first thought it was the passenger sunshade, because of the small accent lights next to the flip up mirror. So, they replaced the shade with another one without the lights, but the problem persists. Any suggestions would be greatly appreciated. I know it is short somewhere, but I have no idea where to begin.