Be gentle.
I'm not sure about this particular scenario, but in general for things like this, make sure the plastic is warm beforehand. Not hot, but like, up high in a warm room for example. It's plastic, it loses its plasticizer over time, becoming brittle, and it will break eventually. Especially if it's near a source of heat (headlight). So basically any kind of plastic that needs to be flexed will just break at some point.
Such is life.