I've tried various things, including the usual suspects, but have had little luck at getting the black colour back. I've put so many chemicals on there at this point, I don't think even paint is a viable option. I wonder if putting the cover in a zip lock bag with something (maybe the 303 suggested??) and leaving it over the winter would bring it back to life?
I've seen some faded plastic parts made to look like new again using a heat gun (basically melt - 'melt' is really too strong of a word - a very thin outer layer to make it glossy again) but it's risky: if not done just right the heat will leave a blotchy finish, worse than it looked before, or even melt bits. I have a broken spare cover around somewhere and I'm going to try the heat gun on it but haven't got around to it yet (just like all the other stuff I need to do that I haven't got around to yet
).