The grill comes off if you gently pry it. There's a few screws under there.
Worst case scenario: you have to take off the door panel. I took mine off a few times, but I don't remember if the mirror mounting bolts were under there or the tweeter. Either way, you have to take the tweeter mount off first, so start from there.
pry the speaker cover off. there is a phillips screw in the center of the speaker housing. remove that screw and there are three 11MM bolts that tighten up the mirror. i just did this last week when my cobra mirrors went on!