I do believe that of all the street versions of the NASCAR makes, Toyota has the largest American-sourced content.
From 2007
"The civilian versions of the three cars that compete in the NASCAR Nextel Cup series the Chevrolet Monte Carlo, Dodge Charger and Ford Fusion are built either in Canada (the Charger and the Monte Carlo) or Mexico (the Fusion). Meanwhile more than 75 percent of the parts used here by Toyota (and 98 percent of the steel) comes from U.S. suppliers....."IIRC there were several articles about it when Toyota made its debut.
As I noted in my follow up post.