I just did this yesterday. The secret is, there is a screw on the bottom outside edge, and you have to get at it from behind the mirror glass. So if you pull the mirror glass out, in the lower outer quadrant, you'll find a sort of tunnel, and at the back of that is a small star-head screw. You need to remove that screw. Then on the outside there's a clip on the inner part of the light which pops right out, and then you can wiggle the light assembly out of its slot.
Then, of course, don't forget to replace the screw when you install the new light. :)
I discovered this by zooming in on the part schematic from the Ford Parts site... Hope this helps.
