Headlamp cover polish?

You might try Parasol.

I used to use some stuff on my motorcycle that cleaned up the windscreen real well and took out most of the scratches. I can't remember the name, but it came it a white plastic bottle and was a liquid abrasive similar to a metal polish.

Blue Magic metal polish might work as well.

What ever you use, I would first try it on a similar plastic surface for fear of making your headlight cover worse.

I tried some Mothers Plastic polish on my headlight and taillight covers last summer for show prep. They really looked nice, it was well worth the few minutes to do the job. But my covers were in very good shape already, so I don't know if this polish is abrasive enough to help with slight scratches or discoloring. But it's gentle and made for that type of plastic so I can't imagine it doing any harm. I'm a huge fan of the Mothers 3-step Carnuba.

Mothers® Plastic Polish