Spoilers - why did they have to go through the tunnel?
Spoilers ahead.
OK, so the idea was to get to Diaz in his black Mercedes to have an inroad to the boss. But why did they have to go through the tunnel do so? Much of the movie is spent on getting the location of the tunnel, then going through it, where Alejandro meets the guy who will then basically be his Taxi driver acting on Alejandros instructions which he gets from the Americans?
Why was all this necessary? Brolin's character already knows about the black Mercedes, and the driver's whereabouts. They could have tracked him from the US going to meet his boss in Mexico, follow him, and have him stop in the same way using a "fake" police check.
It's not like they needed someone (like the crooked police officer ) to find out who they were needing to follow in the first place, they knew all that, because they gave Alejandro instructions what highway to use.