So, I popped it off the windshield frame to better figure out how to tear it apart.:
For those that haven't attempted this, the outer plastic frame is snapped into position by 4 tabs, 2 upper and 2 lower. Easy to remove and can be done while still attached to the windshield frame. Unfortunately, my thumb got in the way and I broke the outer glass.
I have never tried to cut glass so I thought I would give it a go. I used an old piece of picture frame glass and etched it with a Dremel grinding wheel. Even though I had cut into the glass a pretty deep outline, when I broke it free, the cut wandered and didn't follow the trace.
I tried this one more time before giving up on this approach. The Dremel wheels erode down quickly. This attempt also resulted in a crack. I think, due to overheating the glass during the grind. Off to the hardware store. Instead of trying to find a bit to cut glass, I decided to try cutting clear plastic. I bought a small piece of non-glare plastic. It cut easily but it turned out to be opaque/blurred when used in the mirror.
Trial and error.......
The 4th try was the charm
Another trip to the hardware store. This time for clear Lexan. This is a bit thicker than the non-glare and played into the decision not to buy it the first time. 
It cut more easily. I needed to grind it a bit narrower to make it fit behind the tabs in the frame housing:
The mirror cleaned up well, showing a little patina. It snapped back together and back onto the windshield frame. Good-to-go