Trim discolouration inside - rear windscreen

[johnny_quattro]

I've just taken the decision to 'de-tint' my windows and I'm pleased with the result, as the car looks more classy and less pimp. I've now discovered that there's a circular patch of nastiness where the finish has come away at the rear of the deck, behind where the BOSE speakers are installed. It's right in the crevice so quite hard to get at so it looks like a fiddly job.

The location is where the rear windscreen meets the rear deck and it looks to be made of perforated metal and painted matt black? I will go and take a photo of it and post back here later.

Any suggestions on how to fix this and get it looking the same colour?