Here’s an unusually detailed glimpse into the activities of a Mexican informant who was part of the Zetas cartel while working for the DEA: Snitch tells of spying on Zetas. It’s unusual in part because of the generally secretive nature of informant use, but also more concretely because trials are infrequent and therefore informants rarely testify. On the extent to which informant/cartel members have become central to U.S. law enforcement in Mexico, see this previous post: NYT: Numerous Mexican drug informants benefit U.S. law enforcement.