I would model the base outer shell of the headlight, as if it had like glass over it. Make sure this base geo is very high res if you plan on using this as a high poly model to bake from. Then create boolean shapes to boolean out the headlight cutouts, and then a lot of by hand clean up to create as clean as possible geo from there using a cut tool and bridge/fill polygon (if you are using maya, but the techniques apply to any 3D modeling suite).
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