The drug cartels running illegals and drugs have spotters in the mountains to watch the border patrols.
The border patrol does not have as easy access ?
That's what makes it great cartel and drug land as well. Easy to hide, and 'survive' and do what bandits have always done.
The BP already uses horses in that area, and that makes sense. Part of the problems they face is that the cartel have better technology and supply chains.
You could stop the cartel in that area, but ya would need to bring back Pershing, and uses modern tech.