That's a pretty big comment from Trump. It would go directly against the new NAFTA deal -- or USMCA -- that he negotiated. In that deal, 75% of the value of a vehicle must come from North America, a certain percentage of employees need to earn at least $16/hour and 70% of the steel and aluminum must come from North America.
The Big 3 all have factories in Mexico as well as most foreign automakers and Tesla has been teasing building a factory there for years.
Now, with Trump you need to take everything with a grain of salt. What's happening right now is that Chinese manufacturers are building factories in Mexico. He may be looking at ways to put tariffs on those.