[tony20009]

Being able to remove the dark spots/areas from your seats depends on the nature of how they came to be that way. If it's from a spill and it has absorbed into the leather, it won't be removable, though there may be some expensive and elaborate means for a professional to re-dye the leather for you. If it's just surface dirt just about any cleanser with will do, though I would initially eschew things with ammonia or bleach in them as those chemicals may compromise the leather's longevity.

Start by simply taking a dry washcloth and some elbow grease to remove anything that's easily removed. If more work is needed, step up to a damp washcloth, which will remove anything that isn't oil based. Next step, try saddle soap, followed by several "rinses" with a clean wetter-than-damp cloth, for it has worked just fine for years to clean leather. Like all other soaps, it works by bonding with oil and allowing the rinsing action to carry the oil-based dirt away. These are where I'd start. (Ivory soap is also a reasonable cleanser to use in place of saddle soap.)

If that doesn't work, try applying a small bit household cleanser (but nothing with oil in it) to a cloth -- not directly to the car seats -- and them immediately clean again with saddle soap and then a leather conditioner.

The trick here is to use the minimum amount of chemistry to remove the soiling. Leather is porous and will therefore absorb what you apply to it. By following the progression I suggested, you'll at least be certain that your leather isn't absorbing something it didn't need to. You can certainly go straight to a product made expressly to clean leather, but I can assure you it won't do a better job than saddle soap, though it will probably not require the subsequent "rinsings."

FWIW, if you've ever had your shoes shined, you'll h ave noticed that the person doing it cleaned your shoes before waxing the leather. That is essentially the same approach I've suggested above, minus the subsequent waxing.

Best of luck.