Toyohashi-shi in Aichi-ken prefecture is the largest and "main" import/export port for the auto industry in Japan. Go down to the docks and you will see Honda, Toyota, Mercedes, BMW (and nearly every other auto manufacturer) with their signage at their respective ports. On the south west side of the bay is the Tahara-town Toyota plant. Go down there - factory tours every Monday to Friday (except holidays). You'll have to book ahead of time, but I don't think they take any groups numbering less than 10 people, though that may have changed. Lived in Toyohashi for 18 months before moving north to Takikawa-shi on Hokkaido.
FTR I
did talk to the Japanese workers from those factories on a regular basis - I was teaching them English 1-3 times a week. Like I said, the factories aren't void of Japanese, but the ones who are there are all managers and engineers. An extremely large number of worker bees, however, are Brazilians.
http://en.wikipedia.org/wiki/Brazilians_in_Japan