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I don't recall there being any glue between the rubber gasket and mirror body. Just a really tight fit between the gasket and the metal mirror body.
Jason is correct. Usually just a tight fit. Very difficult to remove without breaking. Putting new one in requires lots of lubricant (we just use foaming glass cleaner and maybe a little Vaseline). You stretch the rubber gasket over the new glass, then press the glass and gasket into frame. However, I have seen some people use a little urethane or silicone on the install.