i am a PPG certified painter. i pant plastic all of the time.
the first step is find out if they have primer on them or not. i use a lighter colored fine grit, 600+. sand paper and lightly sane it in a hidden area. if the lighter colored s paper shows primer. then scuff with red scotch brite, seal and paint. IF you apply adhesion promoter over primer this can cause a adhesion problem.

if no primer.
then look on the back side for type of plastic. there should be a plastic type code molded into it. poly propylene based plastic and their mixes need adhesion promoter for paint to stick properly to it. ABS type doesn't need adhesion promotor. .
if you can't see any identify marks just cut a SMALL sliver off and place it in some water. try to force it under the surface. surface tension will usually float all plastic until you force it under the surface. PP plastic and their mixes will float 100% of the time. ABS type will sink.
another difference is ABS type plastic is usually rigid and PP is flexible.

kleanstrip is a quality product it should work fine.

now the kicker.
if they are primed with cheap primer the paint can fall off no mater what you do. i just ran into this on a cheap aftermarket Ford Edge lower tail gate molding the cheap primer peeled off taking the top coat with it.
fast test is wipe the primer down with some enamel reducer or possibly thinner. thinner can attack plastic so i don't like using it. reducer usually doesn't. a solvent based wax and grease remover usually removes cheap primer and usually doesn't attack plastic. if it does it will be less aggressive than reducer.
if it wipes off easily remove it all and go back to the top and find out what plastic they are made of.
also if the priner doesn't have a good film build and it's over PP plastic. the primer can fall off taking the paint with it.

i forgot about flex agent. i haven't used flex agent in over 20 years. don't ever remember having problems without it. decent quality URETHANE paint has enough flex to it so it isn't necessary. if you use it. this makes wet sanding and buffing dirt much more difficult.