Have the vehicles permitted in YOUR name, not her's.
I have been nose-to-nose at a puesto de control with the SSP jefe who got all excited because a line by line, letter by letter audit of car papers and tourist cards revealed not only different modes of transportation but different permit DATES. The guy I was helping flew out of Mexico and they of course COLLECTED his FMM. Upon re-entry by airflight, they issued him a new FMM. You DO NOT GET A 180-DAY tourist card when you fly into Mexico.
The permit had expired, he assssssssumed the new permit was for 180 days. The FMM time did not agree with the fecha the date on the car permit. The federal cop CONFISCATED his car papers, and told him to go to an INM office and get stuff fixed and when he got things fixed go to the local PFP office (Highway Patrol) and "make arrangements to get his car papers back". Thank god they did not impound his rig. He drove to the nearest park (back to my place) and parked it.
The fine at the INM office was one thousand pesos. A trip to the bank and back. He was issued a 30-day FMM and he, his wife and rig had to leave Mexico.
REGULATIONS versus ENFORCEMENT
The SSP officer was strict. Since he had federal Mexican law behind him there is nothing much a person can do about it. And if I may add, at no time was a bribe sought or solicited.
A vertical learning curve.
