Left side shown:
The left arrow points to the part you asked about. The right arrow points to the part pictured below.
The arrow below points to the deck panel which is the piece that runs from the rear window frame to the forward gutter of the trunk opening. It looks like this deck panel turns down and attaches to the lower window frame pictured above.
I don't believe the AMD corner pieces are substructure. I believe they are intended as patches for the window channel corners. I bet it's a pain to join to the deck panel and make it look nice.
If your rear window channel is in really poor shape, then you might want to replace the entire deck panel, the lower window frame piece pictured above along with the left and right corners.
Hope this helps and good luck.
@Kern Dog has probably repaired the rear window channel on one of his Chargers. He might chime in if he isn't too busy.
By the way, AMD did a terrible job packing my order. Both lower corner pieces and the lower frame were out of their plastic bags and rattling around in a box full paper packing material that was not protecting the pieces. Luckily, the only damage was a small bend on one end of the lower window frame. Maybe they pack their international orders better than domestic orders?
I bought the parts as "spares"...just in case; and who knows how long these parts will be available from AMD.
