Although it does seem impossible, it can be done. I posted the same question several years ago and received no responses... I finally figured it out on my own, but now am trying to remember how I did it.
Seems like I had to put the channels sort-of in place, but not connected, then tilt the channels inward at the top and slide the glass (with lower channel in place) down into the channels, then tilt it all back into the door and then install/connect the regulator, and finally pull the channel downward while connecting the tang at the top, then securing the bottom. If not exactly right, this will at least give you a clue as to the approach required??? It is kinda like
a Chinese puzzle...
I had done pickups before, but they are easy because the tops of the doors come off and the glass can be dropped directly in. Those being easy caused me to have
unrealistic expectations on the 5WC...