You can get rid of ash stain out of a car's ceiling upholstery pretty much the same way you remove them from your regular upholstery.
1. Using a clean white cloth, sponge the stain with a dry-cleaning solvent.
2. Blot until the solvent is absorbed.
3. Repeat Steps 1 and 2 until the stain disappears.
OR
1. Mix one tablespoon of liquid hand dishwashing detergent with two cups of cool water.
2. Using a clean white cloth, sponge the stain with the detergent solution.
3. Blot until the liquid is absorbed.
4. Repeat Steps 2 and 3 until the stain disappears.
5. Sponge with cold water and blot dry to remove the detergent solution.
Get more tips an how to clean the interior roof of a car at YourMechanic.com.