Cars are made by attaching multiple parts together. Model it as such and it simplifies the structure of each part.
Same for headlights and other lights. Headlights are made as a unit (reflector type): front glass, reflector housing, bulb housing, and the bulb. How much of that you have to model like it’s built depends on how you shade the thing. With front glass refractions and a light, might be able to cut corners and use intersecting geometry and dummy geometry to get an impression of a more complex piece.