Guys, I spent a great deal of effort resolving some of this mystery, so I thought I'd share the information I have after finding and installing one.
The GM Rear Air Deflector shown on the "accessories" web page DOES exist (Part #12497618), but is no longer recognized by their Parts Department.
This item has been discontinued and the replacement part number (17800651) is the flat blade of transparent smoked plastic with a dent along the one edge to REVEAL the third brake light in the OEM location.
See this thread for more info :https://forums.trailvoy.com/showthrea...flector&page=5
There are some good installed pictures of both (Page 2 - new) (Page 5 - old)
Once I found the old part number (12497618), I called several dealers until I found one still setting on the shelf. They were glad to sell it (free shipping)
ANYWAY here are some details on how this thing installs:
First it is molded to closely match the third brake light applicade and comes with a plug to RELOCATE the third brake light reflector.
The instructions are very easy to follow and walk you through the REMOVAL of the OEM reflector and RELOCATION on the deflector arms and plugging the orignal reflector hole.
The integration of the OEM reflector INTO the rear air deflector really disguises this aftermarket modification and would easily pass any vehicle inspection.
As far an "functional", it does an incrediable job of reducing the amount of "road spray". Even at city speeds (<30mph) virtually no mist accumulates on the rear window. This greatly contributes to a cleaner rear end. No dust, so no need for washer fluid that eliminates mud trails down the gate.
For what it's worth I am very pleased with this mod but be prepared to do a lot of footwork yourself. My dealership wouldn't even help locate the part. They can't order it anymore and that was that...
BTW: Sorry for the pictures. It is winter here and my truck in embarrasing dirty all the time.