Just a word of caution, for followers of this thread: if you're not getting yellowing or some other anomoly on your headlights, don't use these products. Make darned sure it's not just dirt and road grime. The headlamps actually have a UV coating on them. The fact that this coating has broken down is what causes the yellowing. If you have "healthy" headlights, you'll actually be taking the coating OFF if you use these products. If your coating is already SOL, it usually shows itself by "flaking" or "peeling" much like your skin, after a sunburn.
So, again, check your lights out very closely before applying these products, some of which contain petroleum distillates. Thank you -- that is all.