If the grain of the leather is visible on the leather then it sounds as though you have an aniline style (one that absorbs moisture). Stains such as marker stains and paint stains and dark spots will be impossible to get out of a leather like this, but it may be worth trying a leather cleaner (Lazy Leather) to see what reuslts you can get.

The leather has not been looked after correctly and it may be a better option to replace the seat panels with some new leather or fabric.

Hope this helps