Over time, the headlight and turnlight plastic on my 300e becomes brittle and cracks like old leaves. Not only is it unattractive, but the final stage demands replacement so the lamps themselves don't burn out. And that replacement plastic is expensive!

Is there anything anyone has found to treat, put on, or rub into the front plastic to make it last longer?

Bob

Given exposure to heat and cold, the polymers in the plastic are going to age regardless of any external coating, although you might make them look nicer for a bit longer. The ultimate in longevity would be the glass lenses in the euro spec headlight units.