[BLE 'BURBAN]

Jabin Wood can set you up with the under dash harness for the mirrors alone. The factory harness is all wrapped in with the power window-door lock harness.

As for the inner factory brackets, they have long since been did continued by GM. And they are glued in place inside the outer skin and are a real ***** to get off. Also, the bottom mounting bolt is a part of a structural support bar that curves around the front window channel and is bolted to the inner door shell.
If you cant locate these pieces in a yard, you can get some 3/16" thick by 4-6" wide flat stock from Lowe's or Home Depot or a local steel supply company cut 2 pieces to about 8" in length each.
line the front edge of the mirror up with the front edge of the wing window frame making sure it won't interfere with the vent window when opening it.
Mark your mounting mounting holes as well as the hole for the wiring and drill them out with a 3/8" bit. Use a 1" hole saw for the wiring harness hole.

Now take your flat stock and position it inside the shell tucking it as far up as you can.
You can use a piece of duct tape to hold it there temporally.
With a black sharpie color in the mounting hole positions as well as the hole for the harness on the flat stock from the outside of the door.
Remove flat stock, Center punch all holes.
Now there's two(2) ways you can do this next part....
1. Drill the mounting holes to 5/16" for the mounting bolts to pass thru and secure with nuts.
OR
2. DRILL AND TAP the mounting holes for 5/16" NFT bolts 1" long with a bit of locktite as I did.
I applied some JBWELD to the contact side of the flat stock before I reinstalled it in the door and bolted into place long enough for it to set up. while I worked on the other door.
46 minutes later, I went back routed the harness thru, removed the bolts and mounted the mirror.
Then I went back to the other side and finished it as well.