I used to have tan carpet in my 3G, and one day I got fed up and removed the carpet from the car and gave it a good scrubbing with a brush and Palmolive dish soap and plenty of elbow grease, then rinsed it and let it dry. Tan looked so clean it was almost salmon colored very light and no evidence of dirt.
As for leather I've never had leather, but I would probably just do the same, maybe a little degreaser and then hit it with conditioner to get it back to life.
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