Just for fun, this video shows an actual assembly line made from Lego® that actually works and assembles a car. While the car itself won't have an engine (consider this the spoiler), some things from this assembly line seem right to me: the recyclability of Lego pieces, the simplicity of the process (which is, of course, cleaner than a foundry) and the non-pollutant engine system.
Automakers really think of cars as complex pieces that are assembled - and they create economies of scale that allows the same parts to be used in different vehicles and the same assembly line for different models - this saves them money and energy. Not very unlikely on this toy assembly line.
[Source: Wikipedia (Thanks to my buddy Bruce for the tip)]