Japan has three main unions and then smaller unions for specific companies:
Rengo, the largest, represents the private sector and has over 6.6 million members
Zenroren and Zenrokyo represent trades and have approx 1 million members between the 2.
Edited to add: Confederation of Japan Automobile Workers' Unions (JAW) has approximately 770,000 members and an English website here:
http://www.jaw.or.jp/e/Specific companies that might be interesting to some here:
Toyota Motor Workers' Union - 60,000 members
Honda Union - 40,000 members
Nissan Union - 30,000 members
So yes, Japanese car companies in the US are not unionized, but they are back home (I actually have personal experience with this. My wife and I own a Ford and a Honda. The Honda we bought was 100% assembled at a Honda plant in my wife's home prefecture with union labor, which is why we bought it.).
A recent example of union action is the Toyota union dropping a demand for a universal pay raise this year in order to protect seniority based wages (A big part of Japanese companies):
http://www.jihsin.com/news/toyota-union-plans-to-forgo-wage-hike-demand.htmlAnd to my subject line:
This is the most recent data I could find after a few minutes (heh) looking on the web, which, unfortunately, is not all that recent (i.e. why I parsed my subject line a bit):
Union membership as percent of workforce in:
Sweden: 78.0% (2003)
Belgium: 55.4% (2003)
Brazil: 35.4% (1995 only)
United Kingdom: 29.3% (2003)
Japan: 19.7% (2003)
United States: 12.0% (2006)
Korea: 11.2% (2003)
France: 8.3% (2003)
http://www.ppionline.org/ppi_ci.cfm?contentid=254182&knlgAreaID=108&subsecid=900003