best way to fix rust around rear window

Kevin, if areas are flat you can just cut out and use flat sheet metal to replace. If rusted areas are angled with patterns in them it would probably be best to cut that area out of a donor car. Cut off more than you need from donor car to be sure you have enough to replace your rusted area. You can always trim piece down once you have it. Sometimes if rust is near a factory seam, it may be better to go to the seam with replacement metal. You should take some pics and post them here. There are quite a few body people here that can better advise you if they can see whats needed. Mike