Ok, i looked into my model, and the windshield and other windows were indeed just simple polygon planes, not meshes with some thickness. So i extruded the windshield to give it some mass, and yeah, the path tracer liked that a lot better than the version, that was basically just a plane with zero thickness. With that change, the windshield became transparent and reflective, just as you would expect it to be.
So i guess, you should check your windows meshes too 
Sidenote: Give your simple glass material a parameter for metallic too, set this to 1, and set the color to white (or any other bright color). With that, you can create colored glass, if you play with the opacity ->make it less transparent. With metallic 1 and white as color, it will act like one of those reflective sunglasses, if it is not perfectly clear ^.^
Remember, metallic materials need a color input greater than zero to be able to reflect something, white for a chromelike material.