Mexico is part of the North American Free Trade Agreement (NAFTA), a Latin American free trade deal, and a deal with the EU which means its import duties to Europe will drop from 10% (made in USA) to 0% (made in Mexico).
The decision isn’t all about import taxes or duties; FCA and Ford are both moving all possible car production outside the United States in search of higher margins.  At the same time, newspaper recounts claim that the government has actively kept wages down — contrary to the intent of NAFTA — so workers still get paid relatively little.
As long as fuel prices stay low, the United States and Canada will still have assembly jobs: the small and medium size cars are moving out, but crossovers and pickups are taking up the slack, at least for now.  
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