the direct answer is a couple drops of rubbing alcohol on a Q-Tip -
BUT - don't be too disappointed when that doesn't help brighten/lighten the indicator on the fender -
over the years the fiber optic cable has probably cracked/broken in several places -
thus loosing the ability to transfer as much light through it when compared to new -
ALSO - the plastic lens that the fiber-optic cable plugs into has gotten cloudy - compared to new -
Isopropanol, aka rubbing alcohol, is what I was about to suggest, but never tried it on these optic cables. For some reason CH3CHOHCH3 seems to dissolve the plastic on PS3 controllers.
Those signal indicators in the fenders were painted over by a shop when I had her repainted. I bought replacements, maybe the last two in the country, but never installed them. Now I’ve gone and misplaced them... Sigh.