Trump can't think past his nose. Mexicans cross the border illegally because they want a better life. Illegal Mexicans contribute to American life paying taxes, providing cheap labour. Yes very few are criminals. 
So Trump wants to build a wall to keep Mexicans out, restrict free trade and give those jobs (low paying ) that Americans would not do back to them. 
Free trade enriches Mexico and America and Canada. It helps the Mexican economy which helps to stem the tide of illegal Mexican immigrants. I think if Trump thought past his rhetoric and "increased free trade" to Mexico then he'd be further ahead with the illegal problem.