There are two different kinds of visas. The first type is a tourist visa, with this option you are allowed to keep your car in Mexico for 6 months. The other type of visa is a work visa (FM2 or FM3), this visa’s length is determined by the Mexican government and your car is allowed to stay in Mexico for as long as the visa is in effect. You will be issued a sticker and the car will be legal.
Temporary importation is very simple. The process can be done online, at a consulate, or when you are crossing the border. If you have your documentation in order (passport, title, registration, and license) then the process is as simple as promising to bring your vehicle back on the agreed upon date and paying a small fee ($30 or so). If the car is not brought back in time you will be charged $200-$400 depending on what type of car you have and your car may be confiscated.
Permanent importation involves having your car registered as a Mexican vehicle. To do this you must have either an FM2 or FM3 visa or be a citizen. A permit will have to be issued by Mexico allowing you to bring your car in. However, Mexico also has a rule that any ten year old car can be imported without a permit. For example, in 2008 a 1998 car could imported without a permit.
Like most foreign imports you need to hire a customs broker. Theses people are very knowledgeable about the importation process and will make sure the right forms are filled out and the correct fees are paid. They are especially helpful if you do not speak the language.
Vehicle importation is not something you want to figure out as you go. It can become very costly and the process is not friendly to first timers. Do your research and hire a knowledgeable customs broker. And if you don’t speak the language make sure you have someone with you who does.