on heavily dulled/yellowed plastic lensed headlights, i start with 4000 grit wet n dry, then cutting compound and finish off with polish.
don't be scared, they will look like new!
It is slightly abrasive and thats why it works. Its like a super-fine cutting compound. Just wack some on a rag with a little water (you'll get a feel for what works) and polish away.
Just remember to return the tooth paste to the bathroom before the misses needs it while running late for work the next day.
Toothpaste or light cutters works well but the headlights will soon turn yellow again if they haven't been sealed it's uv light that destroys them. You can buy headlight restoration kits from repco burson ect for about $30 that comes with uv sealant we use them at work all the time.
Toothpaste in the holes in gyorock from picture hook nails etc.
Since were on about toothpaste.
On the lights, yeah the restoration kit is nessesary in the end because you just keep on losing the shine. Unless cutting them regularly suits you better.