My electrician said: 1) I should be careful choosing a light fixture as the back may be visible when mounted on the mirror. What affects how much of the back will show? In other words, what should I look for in a light fixture to avoid this problem? 2) He also said that the light fixtures slide around because they can't be tightened down--the mirror can crack if they are over-tightened, and they scratch the mirror when they slide around. Has anyone experienced these problems and/or have advice on installing the fixture to avoid this? Many thanks for any help you can provide!