You will need to select all faces of the windscreen. Now, you can do this one of two ways. Preferably, Ill usually just bake the glass transparent. Or, you can map it and give it the ~ALPHA or ~CANOPY material so you can control the alpha on the .tga.
Either way, do the following: Select all faces of the glass, open the menu, right click "material", give it a name, adjust properties.
If baking, name it anything you like, and adjust the opacity slider in the material properties popup.
If mapping it for tga control, name it ~ALPHA or ~CANOPY and leave the opacity alone.
Below is a description of the material differences.