Mexico is an assembly operation to arbitrage cheap low skill labor and preferential NAFTA-like agreements.

Vehicle designs, powertrains, critical components are all made by the HQ or its suppliers.

Citation needed, because just a cursory check is showing me plenty of powertrain manufacturing happening in Mexico. Meanwhile if by critical components you mean chips, I don't think there's a big semiconductor manufacturing union that's kneecapping GM. Design is also an apples to apples comparison, it's not a union job.