how does the mirror control knob come off 2007 F150
Are you replacing just the mirror or the entire fixture?
the whole fixture
the inside door panel must be removed. That should expose the three bolts/screws that hold the fixture to the door near the window. Once the panel is removed and you see the inner workings, it's straight forward.
Forgot to mention there are usually a couple of screws concealed near the arm rest, and maybe in another location, usually behing the handle to open the door which provides stability for the center portion. If you have removed a door panel before, you will understand. If you haven't, invest in a Haynes manual for your vehicle and it will have pictures of where all the fasteners are.
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