I researched online for the law in the US and feel reasonably confident about that (proceeding without police involvement as I wrote previously). Regarding Mexico, I don't know, the law could well be different than it is in the US. The thing is, regarding Mexico, you could try and proceed according to the law there and still not really have a chance of succeeding ( the info on the US Department of State website that I linked to earlier in this thread leaves little doubt about that in my mind:
http://www.state.gov/j/tip/rls/tiprpt/countries/2014/226777.htm ).
JR could try approaching his employer, Frontier Communications, about the possibility of financial help. I have no reason to doubt the integrity of his Mexican PI, so I would continue to follow his advice, running suggestions by the Mexican PI from members on this Websleuths site and elsewhere. I would try to get the Qualcomm data for the suspect truck, which still looks like the best lead, and to this end I would contact Qualcomm Headquarters in the US (I would ask the Mexican PI and a US attorney about sending a Spoliation Letter to all of the companies involved, though it might be late for that and I don't know if this is an accepted instrument in Mexico). I would ask the Mexican PI about the advisability of trying to use bribes, if nothing else was working. If a search area or locations could be identified with this data, possibly a ground search with cadaver dogs would be appropriate.