Originally Posted by gring0loc0
I've had a different experience. Crossing the border, with my FM3, I say "I have an FM3", and they say "ok, go ahead".
I've never been asked for a permit, and from what I've read from official documents, showing my FM3 with the same name that the vehicle is registered to is considered a permit to use the vehicle.
Well, you can always get lucky. Try getting pulled over by a Federal de Camino and show them your FM3 instead of a TVIP and see how that works out for you.