Well, to be honest it's been a very long time since I've done modeling, and an even longer time since I made this post.
So don't quote me on this because I really don't know if this is right.
But from what I remember, I believe you would need to change the glass ID for all of the [COL] window objects to ID 122... as well as the normal 'Crash' properties on both the object and the glass material (see Chief's reply).
120 - weak glass - side windows
121 - medium glass - rear windshield
122 - strong glass - front windshield - combine this with the 'FLAG_HAS_BULLET_RESISTANT_GLASS' in vehicles.meta and you get, well, bullet-resistant glass, with the proper "blocked bullet" drawable being rendered.
123 - unbreakable/bulletproof glass - oddly this makes it, so every window has the bullet hole drawables when it is shot. However, this does not represent the actual state of the window, it will remain "bulletproof" even if bullet holes appear.
As for windows not breaking... I'd certainly like to know what I thought 6 years ago to make me think I had it working, because I sure don't remember now. Sorry.