How do you get that dirty fabric look?

Airbrush, blended oils, drybrushing, pastels. Any of these alone or combined can get the effect you're looking for.

On the duece and a half, the seats would be leather. I just posted this elsewhere a few days ago, paint the seat the color of leather (I find a roof brown of any paint manufacturer is satisfactory). Then dry brush some silver on it to show wear. A clear coat of flat (I like lacquer) will distress the silver just enough. A little light colored chalk dust finishes it off.

I have been known to rub dirt on the items I want to look "dirty". I then seal it with Dullcote.