Well, for what it's worth, here's what has transpired. I emailed Year One. After a couple days they responded with a near useless answer. They said : The plate is as you guessed. It slips over the control prior to being
installed in the door panel. This keeps the control in position and
makes it so the bezel nut can be tightened.
I hope this helps.
It helped only a little. No direction on how to orient that clip.
After mounting the mirror, I ran the cables between the outer skin and the window track. I couldn't find a place to use the clips that came on the cables, so I used cable ties to route the bundle around the window toward the front of the door. Pulled the control out the opening in the door and jammed that clip thing on. I couldn't get it straight and couldn't get it back off. Anyway, the boss ring on the bezel is thicker than the door panel, so the whole thing was loose. So, I just stuff a little foam rubber between the panel and the bezel.
So, now it works. Yippie-ki-yay.