Right after the bearded merc tells the cab driver, "El Paso truck terminal", the shot of the trucks' passing on the sunny road clearly shows the shadow of the rock-bluff-mounted camera on the grassy shoulder below.
The caballero who's guarding the trailer would not have shut the side-door again after breaking off the lock; he would have known that the frozen cargo would melt faster in the heat if all of the reefer's doors were open.