It was on his platform as he ran for President. To young to remember?
"Mexican President Salinas and President Bush began negotiations for a liberalized trade between the two countries. Prior to NAFTA, Mexican tariffs on U.S. imports were 250% higher than U.S. tariffs on Mexican imports. In 1991, Canada requested a trilateral agreement, which then led to NAFTA. In 1993, concerns about liberalization of labor and environmental regulations led to the adoption of two addendums.
NAFTA was signed by President George H.W. Bush, Mexican President Salinas, and Canadian Prime Minister Brian Mulroney in 1992.
Reagan had nothing to do with NAFTA, perhaps as a senile old man he thought it was a good idea. However he had nothing to do with NAFTA as it was negotiated after he left the presidency.