I could be wrong but I believe it's bounded by E. Montcalm St. on the south, Joslyn Ave. on the east side, E. Beverly on the north and N. Glenwood Ave. running through the center of it.
The other GM assembly plant is on S. Opdyke Rd. bounded by Campus Dr. on the north side. Campus Dr. terminates at the front door of their main plant.
Which one is which I don't remember.