For rejuvenating black plastic trim work, in 40+ years of shade-tree detailing I've yet to find anything as effective as these two products:
Pre-Wow cleaner @ AutoGeek
Black Wow restorer @ AutoGeek
Used as a pair, first cleaning and then restoring, it's fairly amazing how much better trim can look. With a good washing and clay bar, followed by the cleaning+restorer pair, it's really effective. Completely removes old residues and waxes. Dramatically reduces the visibility of "old" haze and oxidation that might be present on the part.
It seems to be fairly expensive stuff, but then a small dot of the stuff goes a
long way. Comes as a 6oz cleaner and the 4oz restorer. Have done three car applications so far, and have well over half left. Have done grille plastic, side mirror trim, bumper trim, and trim work between the windows. What trim gets hit with these ends up looking good for a couple of years following the application, in my experience. A much deeper, richer black once all the gunk has been removed by this cleaner+restorer.
Impressed, despite the high initial cost ($15 for the cleaner, $30 for the restorer). Worth trying, if you haven't been impressed by the various "black restorer" products on the market. Whatever it is they put in this stuff, it's different.