Alpha map should be in the alpha channel of the diffuse I think, haven't tried it yet. For your texture, the windows seem to miss dirt compared to the rest of the vehicle.
yeah, the alpha is in the diffuse alpha slot. but you can see the black borders on the windows. they should be 100 procent visible (not see trough) while the dirt should be see trough. For transparancy I use dither. Wich works but gives some unwanted effects. The black on the windows is a halftone gradient but as you can…