Great ideas on the seats, people!
As for the carpet - IIRC you said the carpet is black (boy, you don't know how lucky you are) and it's red clay that's ground in there. Assuming that you've already taken a stiff brush to the carpet to loosen any more clay/dirt and have vacuumed out as much as you possibly could, I would then - don't laugh - take black liquid shoe polish and go over any areas that need a dye job. Use the stiff brush to help work the color down into the fibers by rubbing it around.
If your wife dyes her own hair get her to help you since the process of systematically covering hair with color would be similar to doing a good job on the carpeting.