I had this problem in my 98 cavalier. My dad and I worked to measure whether or not the rear defrost strips were receiving electricity or not. We were confused when we discovered that they were receiving an electrical charge. When the button was pressed, it made a clicking sound as if trying to start and struggling. Eventually we found out it was simply just the button. It must have been sticking or poorly connected. We just sort of worked with forcing it off to different sides and applying varying amounts of pressure to it and the rear defroster just sort of came back on. It took a lot of playing about with it. We even tried taking off the front panel but still couldn't really get good access to it. I know it may not help with your situation but that's how we remedied it.