I find that the rear glass is much harder to break than the windshields, which can crack if you breathe on it wrong. I've removed several of rear windows from ebodies with a windshield knife, none broke. I usually try to scrape off any hard stuff that might be on top of the gap, just to make it easier to move the knife. Lots of times the old seal is deteriorated in several spots, so the knive may pass easily. If either the top or bottom is particularly well stuck, I usually cut allong the sides, and the bottom or top, whichever is easier to cut, then gently and slowly push the glass out, tilting it on the stuck edge.