Glass texture for car....how?

I’ve been working on a somewhat low-poly car model for source games and the modelling has gone just fine. But i’m a bit stumped about how to do the window glass. I want it to be clear of course but I was also hoping to find a way to make it look dusty…any suggestions? I didn’t really know where to start.

Get dirty on texturing, of course…
If you unwrap glass part, set new texture file and paint some dirt on transparent layer in gimp, photoshop. Nothing fancy, light airbrush, some smudged spots, white, brownish. Or go wild for a muddy track and leave clear only under wipers. Set this file as one of affecting colour or transparency or both for a glass material. Could be several. Pay attention that specularity will be inverse to your dirt, might consider making greyscale texture for that on another layer in gimp.