I also haven't researched but I kind of remember...
...from when my brother-in-law was a trucker that they had to have licenses to do the hauling in big rigs. Those licenses paid for at least some of the monitoring. The Weigh Stations are where the monitoring takes place. I would imagine the Mexicans would be required to stop there, too, and the fines for bypassing can be pretty hefty. I also think they can lose their licenses for that (but I can't verify that at the moment).