I painted the heater vent trim and some of my center console in my 91 Civic Si hatch. Its really easier to remove (if possible) the parts you want to paint. Set aside at least a day to do this (I'll explain later).
1-Wash all parts in warm soapy water well. This will remove any dirt and interior protectant that might have been sprayed on it.
2-Let all parts dry throughly
3-Rough up plastic parts with fine grit sand paper.
4-Spray a few light coats of automotive plastic primer on the piece and wait recommended time to dry.
5-Apply very very light coats of your interior paint. The key is lots of light light coats. At first it will appear that the colour is not right, but the more coats you build up the better the paint will look.
All my painted pieces appeared to be pretty chip resistant, glossy and durable.
The real time consuming part to this process is building up your light coats on the pieces. I spent about 1 hr washing/drying parts and 1/2 hour sanding. The kicker...... It toook me 3hrs to paint all the pieces.