A friend of mine that keeps his cars pretty clean looking suggested
this, it's a bit of work, but I am going to try it on my "new" 99 S 80
with LOTS of black trim.

He said he uses rubbing compound, the type you use for cleaning
oxidation off of auto paint. { In the US, "Dupont #7" comes to mind}

It's a 2 step process.

1. Use the rubbing compoound first, to buff/polish the black trim.

2. You will now have a white glaze on the trim, from the rubbing
compound.
Now use a cleaner/degreaser to remove the rubbing compound residue.
He suggested a citrus based cleaner, the orange cleaners you see
everywhere now. And he uses a stiff nylon brush to clean that off.

I HAVE NOT TRIED THIS YET, although I will this weekend to see if it
works.

It makes sense, though; the sun has oxidized the outer black surface,
so the rubbing compound should remove that top layer that's gone dull.
Of course, the black surface is more porous than auto paint, so the
cleanup of the rubbing compound is necessary to get "back to the
black", so to speak..

I have used the moist towlettes saturated with liquid, I think they
are called "Back To Black" by Turtle Wax , and it looks great after
using them.
For about 1 day.

The Florida sun is brutal on auto finishes, and especially black trim.
It's a constant battle for those of us who do it ourselves.....