Mexican Tourist Permit Is An FMT, Not A Visa
You need a Mexican tourist card (FMT) to Travel Mexico Beyond the border
This is a short excerpt from Live Better South of the Border. Information has been updated slightly in 2008.
You don’t need a Mexican visa to travel to Mexico as a tourist. You need a Mexican tourist card, called an FMT. Mexican tourist cards (not visas) are no longer free (except in Sonora). Although the cost of your Mexican tourist card varies according to the peso and does go up periodically, a Mexican tourist card will set you back about $21 (2007—the cost will only go up). Sonora has a “Sonora Only” tourist card that you need to get, and, theoretically, you need a Mexican tourist permit for Baja, but the enforcement of this rule is sporadic at best. Technically, you can only be a tourist in Mexico for 180 days out of 365. Technically, (but seldom enforced) you try to return to Mexico after your 180 days are up, you could be told to go home. I have only rarely heard of this being enforced. Your car permit to drive in Mexico is another document you will need if you are driving in Mexico beyond the border.
Now, thanks to computerization, the government can keep track of you (conspiracy theorists, unite!). They keep a database of everyone who enters and for how long, Be sure to turn your tourist card in, the Migracíon office near the border. Finding these is a challenge, but worth it. Hacienda kiosks are usually near (but not on) the bridge or border crossing. The toll-takers on the bridge cannot take your tourist permit. Since you have to turn your car permit in anyway, find the Hacienda office and turn them both in at the same time. How, you ask? Ask at the 21 kilometer checkpoint (if there is anyone there). If no one is on your side, I would park, walk across the highway (if there isn't a lot of traffic) and ask the guys on the inbound checkpoint where it is. The next best way is to ask a cop for, La casita por los tamites del auto.