Mix a couple of drops of ammonia with warm water and enough dish soap to make the water sudsy.
Wet a soft cloth in the mixture. Scrub the affected area of the car window until no shoe polish remains.
If spots of shoe polish remain, get a very fine piece of steel wool, typically 0000 or 00000. Dampen the steel wool, and test it by rubbing it on a small area first, checking the glass for scratches. If there are no scratches, continuously apply the water mixture to the stubborn spots while rubbing them with the steel wool until the area feels smooth and appears clean.
Using a clean cloth, wipe the area with clean, fresh water. Repeat this process until no residue remains.
Using a clean, dry, soft cloth, wipe the area until it is completely dry and free of streaks.