I found it funny that a model company copied the faded speaker covers. Looks good Jeremy I tried Autoglym Black stain which did absolutely nothing on them. I used black flock on my 3rd brake light cover, that came out great.
Spent a few minutes today restoring the colour of my faded rear speaker grilles. They'd faded to a rather fetching purple colour (or who knows, maybe they were painted that colour) - but a quick spray over with Plastikote matt black soon had them looking like new again. Another item ticked off the GTV to-do list!