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There is a small jumper harness inside the door, near the pull handle (not the handle to open the door, but the handle used to close the door). This is the jumper harness to the mirror switch to the mirror motors. I believe it is held in place with a push fastener that pushes into the metal door frame behind the interior trim panel.
Sometimes at the plant, they are not pushed in completely, and after a few K miles they can pop out and rattle. Not sure if this is it, but it would be what I would look for first.
In addition, pull the cover off the pull handle and make sure the screws have not come loose and are rattling.