Care to describe your techniques?
Well, I'll try to explain that in English (which is not my native language):
It quite easy to do that job. Nothing special on the outside carbody, but I guess your guestion is regarding the inside-weathering.
It's done with three colors: flat black (Humbrol 33), a light, flat ochre (Humbrol 63) and rust (Model Master 1785DE). The only instrument I've used was a wide bristle brush.
Note: Please check before painting, if your car will survive some heavy treatment with solvent thinner. Some plastics will react heavily when they get in touch with (to much) thinner.
First step: Making a wet wash with ochre and thinner. Place some smaller and greater spots on the bottom of the car. Don't matter if the paint disperses into corners or wherever - that is a welcome effect!
Second step: Place some rust-spots (with only little thinner) linto the ochre-spots. This is done with a steady dubbing the head of the brush onto the car bottom. Do not overpaint the ochre completly, leave an uneven edge around the rust spots. Let the rust color dry a few minutes.
Third step: Use again the head of your brush for dubbing the car interior almost completly flat black. The more often you dab the same point, the more the rust or ochre color will show through the black. You can redo this step as often as you like, until you're satisfied with the result.