Wait is the joke that Cadillacs are GM cars?

It's interesting that other car companies have backed off from the multiple brands/badges. We don't have plymouth, Oldsmobile, mercury, Pontiac.

We have an apples to apples comparison with Mexican-made vehicles, though, since both GM and Toyota build there.

What's the excuse for the shoddy non-union Mexican GM vehicles?

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.