As with all dark plastic, it can fade and get pale marks on it, thus making it look grimey and mucky.
Here is a quick, simple and tried and tested method to restore the plastic to looking as good as new.
What you will need:
1 - a cloth - in the photo i have used a cloth handkerchief
2 - a small amount of oil - i have used sunflower oil today, but cheap veggie oil, olive oil, rapeseed, all work as well
In the photographs, I washed the windows of the car yesterday, and wiped the door and dash, but its left these marks, and i want rid of them...
Firstly, wet the cloth completely, and squeeze out the excess water.
Then dip a corner of the cloth in the oil and work over the affected area. The dish I am using is about 3 ins diameter and its about 1/3rd full...so really only use a MINIMAL amount of oil.
Once the marks have lifted, wipe with a dry clean cloth.
This amount of oil and small cloth has just done the entire interior of the car, and there is oil left over, but the results are dramatic.
And one of the benefits with using vegetable derived oil, is that it leaves no "petrol-ey" smell in the car, like some of the "off the shelf" products...