I would argue that it wasn't unions but rather the manufacturers themselves and the lack of focus on quality and fuel efficiency that lead to the problems. If you look back to 2010 there is a good New York Times article that does explain it somewhat.
This revitalization has little to do with the government bailouts. People are considering American cars because of their improved quality and fuel economy, and high-tech features that are equal to or better than those offered by overseas manufacturers.
In another section.
For the first time since the study was inaugurated 24 years ago, the domestic auto brands posted better scores on quality than imports. Consumers reported 108 problems per 100 vehicles in domestic brands, versus 109 for imported models.
In my opinion this shows that consumers cared about the lack of quality, fuel economy and features in American cars that where present in foreign cars.