Here's a link to a general plastic buffing guideline I have on my own website:

http://www.geocities.com/toyotashawn/colorsanding.html

I've done everything from tail lights to headlights to dash gauge covers to the plastic screen on my dive computer (as shown on the website). Chances are, the plastic headlight lenses on the '92 and newer 4Runners using the 9004 bulbs are just faded and just need more of a buffing than actual sanding.