You are correct. Most automotive moved to Mexico and not primarily for cheaper labor but because Mexico has 50+ bilateral trade agreements with several countries that manufactures and ships material at far less cost than the same product having to go through the US. Estimates placed the savings per vehicle at 1500-2000. US regulations + non existing trade agreements + tarrifs (prior to Trump) is what killed the automobile manufacturing in this country.
If that were true then absolutely NO cars would be made in the USA. My educated guess is the "savings" are less than 10% of that. Corporations keep it a secret what they pay for labor and how much is costs. You have to sleuth around. I think if we knew the truth we'd lynch them all.
My theory is a very small import tariff would eliminate completely the labor advantage of the 3rd world. That is why the go bug f-ers over a small 5% tariff. That small tariff would eat the entire margin slave labor provides.