Green Angels after sundown on anything but a major travel route (cuota).
The -only- thing that works is to let a helpful passer-by call for a grua "tow truck" when they get back into cell phone coverage. Before the ejercito o marinas de armada go home for the evening, they frequently drop off a few soldados to guard you and your rig. They expect the tow truck driver to drop the guards back at their base.
Conversation with a PFP a few years ago...
"Do you patrol at night?"
He looked at me as if I was asking the color of his wife's underwear.
OnStar is nothing more or less than a call forwarding answering service. They need someone to call before the service becomes useful.
I have tried it in Sonora and all over other parts of Mexico. Does not work. I was told it has something to do with the cell service that OnStar uses. Not compatible with Telcel.