[Dan in Pasadena]

I have replaced the mirror glass on my rectangular stainelss side mirror with black RTV silicon and it has held fine.

I replaced the mirror glass in my old 944 which was a flag mirror like yours, but it was not heated....so not sure if it would work for you. On the flag mirror I used Liquid Nails Heavy Duty Construction Adhesive. Comes in both a squeeze tube and caulking gun style tube. That stuff is unbelieveably strong. It is NOT the crappy basic stuff they make for old fashioned 1/8" cheapo "wood" paneling. Again, not sure how it would work with the heat, but I'd be surprised if the heat was hot enough to do anything to it.