So I am re-painting the Miata, so needed to take apart the mirrors.
The prescribed method is to work a knife around the interface between the black plastic and the mirror body and then removing
the mirror and black trim as one step. Like this.

This requires quite a bit of force, so it is difficult (impossible) to do this without accidently cutting into the black trim or the
plastic body of the mirror with the knife at some point and damaging the part. So here is an alternate method.

1. Remove the mirror from the black plastic trim first using a knife. (about 1 minute)

2. Once the mirror is free, on the bottom of the mirror, work the knife over about a 3" long section. This way if you end up damaging the mirror/trim you are doing it on the bottom.
Then using the knife, pry up on the black plastic, and once you have enough to grab onto with your hand, pull the black plastic off at an angle. It seems to pop off in chunks as the glue gives way.





3. As always, YMMV but I was able to take these off without any damage. Upon reassembly, I used clear silicone, but I believe any reasonable glue would work.

 

my bad