This worked (and still works) for me - use a soft brush so you don't scratch the plastic, warm water and Dawn diswashing detergent, and scrub the shroud clean: be sure and get into all the corners and creases, then rinse and dry. Repeat if you have to to get all of the crud and residue off. The Dawn (and elbow grease) might get the crayon or grease pencil markings off the shroud as well. After it dries, coat it liberally with Armor All - you can use clear Detailer's Advantage, or even the clear Tire Shine (although Tire Shine takes longer to flash dry, and may initially look a little greasy). You'll be amazed at the difference and how close it looks to a brand new GM shroud, I know it sound overly simple, but I've done this on a couple of used shrouds that looked like junkyard rejects, and had them turn out like new. The Armor All also has the benefit of restoring a little flexibility and protects the shroud surface.
When you are as income restricted as I am towards my hobby, you have to make do sometimes. The effort is worth it -