This answer is best answered by the paint choice your using. Some plastic paints do not require any special primers, others do.
The most important thing that you must do, which you most likely do anyway. Is to clean and slightly ruff up the surface with a Scotchbrite pad to provide adhesion for the paint, regardless if you prime or not.
The best thing to use is vinyl dye, not paint. And if you use the dye you can get that at any auto parts store, no priming required. Your parts just need to be clean and debris free.
Thanks big help
Plastic does need a primer for plastic; two coats of finish paint will stick, until it is disturbed.