I have had good luck with single edged razor blades. You must be willing to: 1. Not go much past 30 degrees off the glass with the back edge 2. Throw out a bunch, as soon as the get dull pitch it. A dull one will likely scratch the glass and 3. Be careful to not cut yourself.
The rear window will lift out if you want to work on it flat. You need to ope the window and remove the arm acorn nuts from the window frame. Then lift the window past parallel with the ground and the top hinge will separate. HAVE A GOOD HOLD ON IT!! or it will drop on the ground and may break!