If the ghost is only an unevenly dull reflection of the surface (outside surface of clear-coat), it should be entirely possible to restore it using paintwork correction techniques.
(If it's actually an unevenly faded basecoat color due to UV exposure, it may not be easily correctable. However, Porsche is known to use very good quality basecoat+clearcoat paint on these cars, I'd give the paintwork correction a try!)
You can get as fancy as you want for this. One-stage, two-stage, three stage, by hand of by machine, are all options...
Here is an example of a machine-applied, three-stage* paintwork correction process I used to remove scratches on a hardtop:
What's your favorite polish?
For a 2-stage paintwork correction option, skip the compound step, and just go to the finer polishing step. It will work Ok if the scratches aren't too deep.
For a 1-stage option, use an "all-in-one" wax+polish product. Sometimes these are called "cleaner waxes".
(*Note that I'm not counting all the steps, like the initial wash, dry, optional clay bar step, etc. it doesn't mean they don't matter, or it's safe to overlook them, though!)
As you might imagine, the more involved process you throw at it, the better the achievable results are. You may however be perfectly satisfied with a simpler process. No need to overkill.
Of course, you could have a professional detailer do the work. I actually think it would be pretty easy, but the risk is if you restored the paintwork of just this one area of rear deck, and make it really nice and glossy/perfect looking, you may feel inclined to continue on and do the rest of the car, to make the rest of it "match"!
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