If the wiper tray is dry and cracked, use luster on it(yes that cheap green sachet one) because it is very oily and it gives the plastic back its lost oils. So let the luster sit on the plastic for a while, then clean it off with a cloth and apply any good protectant. Or you could use the homemade concoction Ahmed bhai used on his Bilori's dashboard when it was handed over to him. Dont remember the recipe so you can ask him. That recipe's main function was to restore the plastic by providing oils back to the paint.
I apply luster on the plastic part of my wheel arches because it provides an oily surface which is easy to clean off.
Rest you did a very nice job on the engine bay, looks like new:).