Rear glass

Haven't done windshields, but I used butyl rubber many places on my trailer. It sticks better than silicone and stays flexible and gooey. You can clean it off with ethanol. I understand that is what is used on newer (since 1990's+) style "glue-on" windshields. For an installed windshield you are trying to seal leaks, there is a "flow-able silicone" at auto parts that works fairly well.