Ben---you put your finger on why boycotts are a complicated deal. Many GM cars and trucks are built in Canada (Oshawa, Ontario), Ford builds a lot of their products in Mexico, and Chrysler is wholly owned by Daimler-Benz. I guess it's a good thing Japan is in the "coalition of the willing"--and they build most of their cars we buy right here in the US
Also---many foreign-made goods aren't wholesaled in the US by the manufacturer, they're sold by a US importer who takes a piece of the sales revenue. A mark-up in other words. Boycotting those goods hurts a US company too. And in the case of VMC hooks, you gotta make a choice whether you want to have Minnesotans lose their jobs just to stick it to France.
The best way to hit the French in their pockets is not to go there unless you're in business and not going is gonna cost you your job.