You slip a flathead screwdriver into that little divot in the bottom where the mirror meets the glass. You slowly tilt the screwdriver forward, and slowly pull the mirror junction upward.
to put the new mirror in, you will need a small hammer and a piece of soft wood. you can slip the new mirror on halfway, but you need to put some muscle in to get it all the way on. so you position the piece of wood at the top of the mirror junction, and hit that wood with a hammer to drive that mirror into position.
Careful if you do this, I had a tint shop take mine off to apply a sunstrip, they cracked the winshield all the way down the middle. They said they're never doing a ford again as it has happened quite frequently. May not happen, just saying, heads up!
Thanks for this topic. I just bought an '06 GT last week, and the interior mirror was bad (the reflective material). Ordered a new one and installed it today. I was a little leery after reading the windshield can crack (especially since I just had a new one put in yesterday), but following these instructions I didn't have any issues. Whew!