Since the parts you are likely to paint are made from hard plastic, the prep is quite simple.
pull your pieces off the truck and clean them with regular household rubbing alcohol.
then give them a few light coats of primer (white for light colors, greys for mediums, and flat black for darks) or any combination you want to get the right "texture" or color tone.
once the primer dries you can use krylon in a rattlecan. gloss colors wont fade if you also use a spray on clear coat, flat black however must remain flat to get the right effect, so you must redo it every few years, but that's ok because it's just a quick wipe and a few finger presses.
I wanted a sort of " dark gunmetal " accent inside so I just hit a few pieces with cheap walmart flat black $.98cents a can
there's some pictures of the door panels in my album if you want to check it out,
I just flat blacked the part behind the door handle, the window button cover, the top corner triangle pieces, and the little plastic part that surrounds the door lock at the top
I'm going to do some of the center console at some point as well... something tasteful, not to vulgar or aggressive, just complimentary accents, maybe mixed with a little body color here and there. But I rather like the grey interior with the leather, way better than sand tan.