The people that cross illegally to the US are not auto workers from Mexico.
47% of Mexico is “middle class.” That means kids in school, car payments, barbecue on the patio, vacation, etc. Those are auto workers. they are not coming to the US in the back of a pickup. Thugs, wanteds, drug dealers, etc. is what we get from Mexico.
The people crossing illegally into the US are economic refugees. Decimate the auto industry in Mexico and the pressure on the border will be that much worse. That's all. Not a threat, an observation.
Mexico is not a good neighbor of the US. It never has been. The only good thing coming out of Mexico is Corona. Build the wall.
If you destroy their auto industry via a trade war they won't be auto workers any longer.