I asked the question for 2 reasons.

1) if the windshield already had some kind of factory tint or whatever that already totally reflects heat, UV, etc. then adding a film would be redundant.

2) in the BMW world, there are known problems with using tint on double pane windows because the tint alters how the glass was designed to operate in regards to thermal regulation. I didn't want that to be the case. But I guess that does not apply to windshields.

I suppose I'm over thinking the whole thing.