Long-Standing Support for Free Trade with Mexico. Ronald Reagan first proposed a free trade agreement between the U.S. and Mexico in his 1980 presidential campaign. Since that time, The Heritage Foundation is proud of the role it has played in articulating President Reagan's vision of free trade in Latin America and around the world.http://www.heritage.org/research/reports/1993/11/em371-the-north-american-free-trade-agreement
Saying it is one thing. Proving it is another. The only way I could see that happening is if someone compared the texts of the original FTA with Canada, and NAFTA itself.
If any significant differences are found, then one could argue that Reagan might not have approved of the latter. If none are found, then one can infer that Reagan approved.
I have yet to see anybody establish the former.