Mexico lacks the capability to control things effectively, even if they wanted to (they don’t). Drug cartels have executed something like 15 candidates for office so far this election season.
It is typically not long before a new policeman, mayor, or border guard gets offered “plata o plomo” (silver or lead) - take the bribe or take a bullet. In border cities like Nuevo Laredo and Reynosa, you can hear firefights fairly routinely, including automatic weapons.
Going into Mexico is harder and more expensive than it would be for us to build solid barrier in the cities; with patrol road, lights, cameras and sensors (alarms).
If Trump gets nothing more for the wall than $1.6 billion per year for the next two years (about the worst case, unless Dems take the house), then he will pretty much button up the urban areas on the border, where the biggest flows are now, during his first term. That would be about 100 miles of high-end urban barrier, with patrol road, lights, cameras and sensors, or 150 (or more, depending on terrain) miles of strong rural barrier with patrol road and sensors.
The last funding bill for FY18, is going to do two solid layers for all of San Diego (14 miles x 2 layers), 20 miles on the West flank of El Paso, and 33 miles to close off McAllen (where the highest traffic is today). I would guess that next year they will do Laredo and El Paso, and more. After those two cities, the rest of the border is smaller cities like Eagle Pass (Piedras Negras), Nogales and Yuma. Higher priority rural segments (which are cheaper) like Eastern Arizona, might start next year, but likely in 2020.
If that is “all” that Trump accomplishes, it will very strongly improve the Border Patrol’s ability to control the border. If however, President Trump is re-elected, with $1.6 billion per year through his second term, or if he can make a deal for for more up front funding, then all of the urbanized areas, and long stretches of the rural border, will be strongly controlled with infrastructure (barrier, roads and sensors).
It's not like the whereabouts of drug dealers live is unknown!