i have had mixed results with plastic headlight rejuvination.
last summer i did the headlights on my '94 ranger (my dad drives it now) and they cleaned up %200. depending on how dull they are you may only need to polish them...
http://www.therangerstation.com/tech_library/headlightlens.htm
the link above uses chrome polish; i have never tried it only the following worked as a personal experience.
a good soft cloth like an old cotton t-shirt or baby diaper works best;
1) i first used rubbing compound working in slow circles just like waxing - not real hard cause it WILL SCRATCH! practice in a small spot if you are not sure.
2) buff clean - next step is polishing compound. same process as step 1
3) after steps 1 and 2 make sure all residue is off and look from a few steps back for any improvement. it may take a few times to make a difference depending on how bad the lights are.
this did NOT work on my '93 and i had to replace both headlights.
i use Meguires Carnuba as a final step to keep headlights (and my Ventshade drip visors) shiny and bug resistant. some people don't like carnuba but it works well on black trim and window seals and i have been using it as described for almost 12 years with good results.
if you can find an old headlight to practice on first it will show you what results to expect like i said IT ONLY WORKED ON 1 OF MY 2 TRUCKS.
THIS PROCESS WILL SCRATCH THE PLASTIC so be sure before you commit yourself to this.