I found a post like this awhile ago. I copied the text from the post so I could fix mine after my bad crash.
Here it is....
I figured out a way to make the black plastics shine again, not factory perfect, but not too bad either. I'll share what I did and you can try yourself. I take no responsibility though! Ok, for starters, I work in the plastics industry so I have a bit of background on what to do. Most plastics are very pourus in nature. When you see haze in the finish, it's mostly because the pours are open and filled with some sort of contaminent. To make it shine, you need to smooth the surface so the pours are closed. The next thing is, the stock glossy finish was created under heat in the injection molding machine. Recreating that is very difficult, but you can get somewhere close to it, at least better the scratched up. The main thing is to take your time, plenty of it. Use the right tools and patience.
Mother's PowerBall, small one
Mother's Cleaner/Wax - important, it has a very fine compound in it, this is what smoothes the surface and brings out the shine
1000,1500,2000 grit sanding papers
microfibre clothes for hand polishing etc
Ok, if you are going to remove deep gouges and scratches here is what you can try. First wet sand the area with 1000 paper until the area is smooth. Use new areas on the paper often cause it clogs quickly, keep it wet too. Then use the lighter grit papers until the area is really smooth. Again, use water. Take your time and use less pressure with your hands towards the end. The plastic scratches very easy. Then clean the area off with a rag. Take the powerball and some cleaner/wax from Mothers, apply it to the area and slowly rotate the powerball with the drill to work the compound around and not have it spray all over. Gradually increase the speed of the drill. Apply medium pressure, but not too hard. Work the powerball in circles over the area and back and forth. Wipe it clean and apply new compound. Take a rag and apply some compound by hand and then lightly buff out. See how it looks. If it's cold in your garage, you can use a hair dryer to lightly heat the affected area. Lightly buff back and forth with a clean rag. If the finish isn't good enough yet, repeat the buffing steps again. The key is time and patience. For the final finish, use a good quality wax, like a carnuba and apply with light circles, (a small amount) when it's dry, lightly buff. I did this to my quad and it's not too bad, at least compared to the major scratches and gouges I had.
I would advise you to try it on an inconspicuous area first. Also get more advice from people like Mothers and Meguiar's. When I finish my ride this weekend, I'll post up some pics of what I did. Note: you only want to this proceedure when your plastics are scratched, nicked, gouged, hazed or otherwise. Don't do this to try and better the factory new finish.