>>(*) accidentally, I had my own polarized glasses at hand, so I could check - when looking at a pair of IMAX goggles at 45^o to the plane of my eyes, one of the... eyepieces should be completely dark. It never gets completely dark, no matter how I turn them - which is not the case with any other polarizing filter I tried, they all look completely dark when I put them so that their plane of polarization is horizontal (the shades you can buy are oriented vertically).
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>So you're saying it's ... magic ? ( I am a science dolt :-)

Actually it's quite simple, once you understand how polarized light behaves. Just imagine that a light wave coming at you can oscillate not just up/down or left/right, but under any angle imaginable, i.e. any combination of these, shifting all the time. Now polarized light has this filtered to just one plane, as if having a grid, so only oscillations that are parallel to the bars in the grid can pass, the others bang on the grid and get nowhere. Polarizing filter is this grid.

Just buy a pair of polarizing shades and have fun. You should be able to eliminate the glare and actually see through the windows of the car in front of you, no matter how much of the sky is reflected in it, because the reflection (off non-metallic surfaces - glass, paint, liquids, tars, sky) is polarized at a nice angle that's exactly at odds with the angle of your shades, so they eliminate it. For a bit of a weird experience, some pieces of translucent plastic may come in rainbow colors - which includes the glue that holds the layers of rear window glass. I'm running my third pair now, and they're still fun :).