Try fixing the rear defogger strips, find the gaps in the strips. There is a kit, Loctite Quick Grid Rear Window Defogger Repair Kit part no. 15067 of special copper color paint, works well, I have had scratched elements in other cars. I wonder if it's sparking, although if you can see the nick it probably would not. Also, check the connection wires to the rear window, maybe one got broken, sometimes the paint can fix it. Note: I am not sure if the defroster stripes are on the glass surface, if so, you can fix it.
