Vehicle windows are defined in the physics properties of the window collision. See window_lf.col in adder.yft for example.
Enabling Is Glass Window will cause the window properties to appear. The only property you should have to change here is the Window Material. This is the material that the corresponding window mesh uses. Looking at the window_lf mesh materials, we see that it uses two vehicle_generic_glasswindows2 materials.
This is because one material represents the inner glass and the other represents the outer glass. Go into Edit Mode and select the vertices of each material to see which one represents the outer glass. In this case, it's the material without the ".001" so that's the one that the glass window should reference.
That's all that is needed for working vehicle windows!
You will notice that all vanilla vehicles have "shattermaps", which is an image that defines the border of the glass-breaking pattern. These aren't 100% necessary for working vehicle windows but will result in better-looking glass shatter patterns.
In Sollumz, shattermaps are represented as planes with a single texture. For example, adder.yft has an object called windscreen_shattermap which is parented to windscreen.col.
These are always low-res bitmap greyscale textures. Currently, Sollumz has no tools for creating shattermaps, so your best bet is to copy shattermaps from vanilla files and work off of those.
