I gotcha on not seeing a hole for the top mounting from the outside. From re-looking at the first pic I posted and a screenshot (below) from a video I found on line, it looks like the upper part of the channel is mounted to the door from the inside where you can see the indentation on the door edge. Makes sense why the original pic showed the mechanic with a tool pointing to the upper mount location from the door interior. An adjustable viewing mirror would help put eyes on that interior spot.
Note: The video did not show the actual channel removal or door interior view.
