I have successfully fixed the mirror (I hope). J-B Weld Steel Stick placed into the stripped hole, then gently turning a screw into it before it fully cured to create threads. I let that cure then carefully glued the post to the mirror housing with J-B Weld Kwik Weld (it sets up much faster so I could quickly apply it as it became a firmer paste-like consistency. Then re-attached the screws. It's very solid.
For those attempting a similar fix, the video Paul linked to a Cadillac mirror fix was indispensable. Getting to the three hidden screws behind the glass that holds the motor is something i never would have attempted with the glass still in place. Also, accessing the two screws that hold the mirror housing on requires a special screwdriver that can bend. And "man-handling" the mirror glass to its limit as the motor clicks is scary. I'm amazed that little motor survived but it did.
It appears the Eagle chrome electric mirror body is the exact same as a 70s Cadillac Eldorado (and others) to where I would search for those parts if i became desperate. Instead of the AMC embossed logo it has a Cadillac logo which is too bad but I'll bet the glass and motor are the same. The post is definitely different without exposed screws.