It is possible to legalize a vehicle at the border, however, our truck was a model year 2000. You could only Mexicanize (legalize) vehicles with model years before 1997 at that time. So basically, our truck was too new to become Mexican at the time we entered México.
But for those bound and determined to legalize their vehicle–
One method of renewing the permission is to drive back across the border into the United States, have the permission sticker removed, then drive back into Mexico and obtain a new permission sticker. As we lived several days drive from the border, this wasn’t a feasible option for us, although I have heard of persons making the trek every six months for this purpose.
There is another option, what seemed a loophole in the system to us, was the fact that as long as I was in the country legally, the truck was legal. It seemed what we needed at the time. But then seems are always too good to be true here in México.
So this second process entailed taking the original and copy of the truck title, the original and copy of the importation permission paper given at the border, and the original and copy of the page with my picture of my FM-3 to the Aduana (customs office). The nearest Aduana to us was in Querétaro (about a 2 1/2 hour drive). It’s on the same street as the Kellog’s factory and the ejercito militar (military), so when looking for directions from native Queretarians, ask for the road to one of those as no one seems to know where the Aduana is unless you are standing in front of it.
The Aduana was open from Monday-Friday from 9 am until 2 pm. When I arrived, I signed a big book stating my purpose and left my id (passport) as a guarantee at the gate. We couldn’t drive into the Aduana as it was a restricted area (heavily guarded by a man nearing retirement age and a large rope to prevent cars from passing). My son wasn’t allowed onto the compound, so my husband, my son and the truck had to wait across the street while I tackled this alone. I walked about 1/2 mile to the very end of the compound. The Aduana is in a large gray building. I walked around to the front door and signed in another book to pass into the office.
The customs agent had me fill out a letter saying to the effect that I owned a vehicle (blah, blah make and model) and was in México legally (had my FM-3) and, therefore, was entitled to own the vehicle in México until my FM-3 expired next year. I filled out the form in triplicate and signed it. Then we walked to the front of the building to have the documents stamped with the date. And that was it. Really!
However, the fly in the soup was that although the vehicle may be in the country legally I did not have permission to drive it in the country, as we learned several mordida (bribe) stops later. For that, the vehicle must be legalized and issued a tarjeta de circular (permission to circulate in the country). For the make and model of our vehicle, it would have cost more than the vehicle was worth which and for us was not worth the effort. We ended up selling the truck and bought a made-in-México vehicle, known fondly as Butch, and relieved ourselves of that particular headache.