It was actually pretty easy... Went down to my local auto paint supply and bought a can of PPG's 'Acryli-clean'(cleaner,wax and residue remover), 1 can of 'Bulldog' adheasion promoter, 2 cans of spray mixed in my color to my paint code (PS2). NO primer needed.
Clean part(s) with Acryli-clean (or comprable) , mask off.
Pay attention to detail when masking. Pre-mask as close as possible with just tape, then go back and tape newspaper/kraft paper to that. Take your time. The key to keeping the paint on the plastic is the adhesion promoter.
Spray 2 light coats of Bulldog and let dry. Spray med. /light coats of paint about 10-12 inches away overlapping with each coat, not too heavy. Let the paint become 'tacky,' then re-coat. I put 6 coats.
BTW- If you paint the parts on the truck, after you mask with paper but BEFORE you paint, apply wax to remainder of truck and do not wipe off. Do NOT get any on 'to be painted' areas. If you do, re-clean with cleaner. This process will facilitate any overspray that might settle on your truck while you are painting. Wipe of wax after removing masking (when paint's dry), stand back, have a beer and admire your handiwork. Don't forget to post those pics!!

Sorry for the long post, Jim.