It'd be interesting to see some research on that. Most of the plants I'm aware of aren't actually in Detroit proper, but the metro area.

A quick glance over the list of GM factories[1] shows just one assembly plant in the city. Chrysler has a bit more going on[2] in Detroit. Of course, this doesn't take into account all of the suppliers, which is a huge omission. In my experience growing up in the area, it wasn't so uncommon for someone to be working for a company whose sole customer was one of the Big Three.

[1]: http://en.wikipedia.org/wiki/List_of_General_Motors_factorie...

[2]: http://en.wikipedia.org/wiki/List_of_Chrysler_factories