Reports at the time were $100 million+ was stolen, and one of the drivers was inside and sleeping in the truck when it happened.
Today, an indictment against seven men was unsealed:
The court filing does not say how the defendants gained access to the trailer
...
In the months leading up to the Brink’s heist, several of the co-conspirators allegedly pulled off a series of thefts in San Bernardino County, the indictment said. In some cases, the crimes relied on tactics similar to those later used at the Flying J truck stop.