I can confirm that if your car did not come with a dimming mirror, or Homelink/sunroof buttons, the harness connectors are likely NOT THERE.
I was installing an OEM boneyard dimming mirror. I have pinned and tested ALL the wires in the overhead for a reverse /backup line (mirror disable) and it’s just not there in my 2018 LT/V6. I have been inside boneyard impalas that DO have the dimming mirror, but the tail/connector is missing on my harness where it should be.
I could not find accessory switched 12v power in my console, only continuous 12v, Lamp power, or 5v switched. I had to get a "fuse tap" from Autozone and come out of accessory switched fuse #6
, run a wire up the front pillar, and across the headliner to get to my mirror. Also had to dig out a Green w/white stripe Rev camera feed at the drivers side floor (1 of 3 same color) for mirror disable. What a PITA.
Apparently there are "steps" in the option codes that dictate different harnesses. ie: If one of say three particular options is to be installed at the factory, then a standard harness that supports all three is installed. Somehow they save money omitting those few connectors and copper, yet it must cost money to manufacture and keep multiple versions of harness on hand.
Here are the 4 different roof harnesses for my impala. Looking at a picture of them, they COULD connect to a master connector somewhere at the pillar heading down to the main block. This would mean pulling the headliner to install this harness IF it works like that. I don’t know.
22739533 Wire Harness W/ O SUNROOF, W/ O ONSTAR $71.74
20977195 Wire Harness W/O SUNROOF, W/ ONSTAR $78.36
22739536 Wire Harness W/ SUNROOF, W/ ONSTAR $87.48
22739535 Wire Harness W/ SUNROOF, W/ O ONSTAR $78.24
This site is actually run by a brick/mortar dealer in Ohio, where I get my parts and pick them up without shipping costs.
You can find the picture of the harness here.
That said, I cant imagine the homelink option needs anything other than accessory power, maybe even unswitched power?