This other older topic (Window reflection/sun) explores this a bit. After I posted above about possibility of a polarizer film of some sort on the window I got the idea to try putting a polarized sunglass lens in front of my cam.
This seems to work for my application where the cam cannot be pushed flat to the window. I have to tilt it down to view from 3 stories up into parking area.
My first test is promising. Jump to other thread and scroll down to see some early before & after pics.