The border has traditionally been wide open away from the cities. Migrastion back and forth was common and tollerated, because the workers normally returned home to Mexico. As, of course, "rich" men. Prefering to be a big frog in a small pool over being just another frog in a giant lake.
Last few decades it all changed. The foreign Governments made conditions at home a living hell, rather than simple a lack of modern (you know, electricity and running water) conveniences. Not to worry , tho, Obama and the Congress has promised to make America a living hell , so many will go home to be a big frog in a small, uncomfortable pond again.