Originally Posted by dsb

Buy MB touchup paint from the dealership. They will most likely have your exact color in stock. Then, use Etyu's suggestion regarding a smaller tip brush. I don't have any specific recommendations on how to paint it. You'll be making short brush strokes and don't worry if you're not an expert. Again, it will probably be hidden due to it's location and angle.

Rust would take some time and besides... does plastic rust anyways? I don't think you have to worry about that.