Do you mean you using one material for the car and glass on one object?
To have semi transparent materials you need to use the Translucent blend mode, which won’t be right for the other parts of the car obviously. So if this is all one object, it would be best to have separate material ID’s on your model so you can apply both materials.
I think it could be possible to do it the other way, but there is problems with fully opaque translucent materials apparently.
For making the glass you could look at the glass material in started content.