I resolved the rearview mirror issue, thanks to everyone's help. On my 2015 1LE, the swivel base screw is a T-15 Torx. There are two plastic pieces to remove, one is a cable type snap on clamp and the other is a plastic cover which hides the 7-pin electrical connector underneath. The only tool you need is a T-15 torx screwdriver as the plastic covers just snap off by hand. The cable with the 7 wires is held under the cover by pushing into a path built into part of the swivel base arm assembly with the mirror. The connector slides on to the swivel base so once you get that off, the mirror is only held to the car by the electrical connector, so support the weight of the mirror assembly with one hand and don't let it hang by the cable. While I had it off, I called the dealer Part's Dep't and the mirror assembly I ordered was GM P/N 233189285 based upon my VIN. From the Dealer $260.13, from GM Parts Direct $169.35.
So while I wait for my new mirror, I cut a piece of plastic mirror for temporary use that I picked up from AutoZone, cut to shape of the tinted area that is messed up as you can see in the photos attached. In the one photo of the mirror that I am holding in my hand, note the reflection of a ceiling exhaust grate part of which is reflected in the damaged zone and one corner is reflected in the clear mirror. That is really poor rear view visibility !! The temporary plastic overlay will do until the new mirror assembly arrives, but you don't want to use that long term. The disassembled parts shown in the one photo is the mirror assembly with female connector pigtail and swivel base. The other 3 items are the T-15 screw, to the left of that the cable clamp that snaps onto the mirrors arm and to the left of that the longer plastic cover that snaps onto the swivel base and covers the electrical connectors.
Update note: The original posting indicated that the torx screw involved was a T-20, this was incorrect the actual correct size is T-15. The replacement mirror was received from GM on 12October2020 and that was when I realized the error as the new mirror assembly was delivered completely ready to install with a new screw already in, the connector half and cable clamp already affixed to the mirror. The entire install took less than 5 minutes. I just wanted to set the record straight. So please disregard the file name on the picture below, the true size of the screw is a T-15.
Thanks again for the assistance
