Get yourself some cleaner, take a razor blade to the old spot. Then after that take a white scrub pad after it to remove any excess glue.

Oh yeah, forgot to tell you before you clean off the old spot to use a magic marker and on the outside mark the lines of where it once went.

I have always bought the glue kits from Wally World. Take the glue and apply some to the wind shield and some to the metal backing on the mirror, which hopefully you have removed to make things easier. Then place the metal tab in place and hold that sucker there for about 1 minute maybe 2 depending on if you can hold your arm up that long. After that let it sit for a little while and then attach the mirror to the tab and then presto change-o you mother fucking done and off to drink beer with me again at the Ale House .