I used to buy Simoniz 'black chrome' wax polish, but haven't seen it around recently. Carplan black trim wax is the nearest I can see and have used it to do all the black / dark trim on the car (roof strips, window wiper arms, exterior area in front of windscreen and strips along lower edge of side windows etc.)
It is actually a wax with black pigment in it, not to be confused with 'back to black' and 'armor all' sprays which are just a clear silicon based liquid with no colour in them - these will put a shine on things that are already black, but will not restore colour.
I have used an 'armor all' semi-matt finish spray on the dash of my Civic and wifes Jazz, and it does give a more pleasing, less glaring finish to the plastic.