When a foreigner brings a vehicle into México, Aduana (Customs) issues a vehicle importation permit.  This permit is usually "valid" for six months and an expiration date appears on the window sticker.  The law now states that this permit does not need to be renewed when it expires:  it is valid as long as the foreigner mantains a valid visa (such as an FM-3 or FM-2).  Authorities in the larger cities are generally aware of this revision in the law.  In the event, however, that you may be travelling in rural México and some local official is not aware of these changes, there may be a problem with your "expired" window sticker.  We suggest that you may wish to copy (remember to change your printer to "black and white") and carry the following documents in your vehicle.