You need to get your FM3 before you get into Mexico.
So you decided like me to get an FM3, which allows you to stay for more than six months, and keep your car in the country if you brought a foreign car in. the following information will probably save you hours of frustration. The following applies to a non working FM3. You could hire a local agent that will do it for you (price range $80 to $ 150). The only thing is that those agents usually delay your application as they wait to have a whole bunch of those before they go down there. The other solution is to visit the following link www.inami.gob.mx. Photocopy your passport( all pages including blank ones)and bring the original with you ( to compare). You need a proof of residency a rental contract and a utility bill, phone electricity… you must supply 3-4 bank statements with your name on it. They are typically looking for about $ 1200 per month for two people.
Good luck, with your application, ans “Hasta la vista”.