If you've been looking for good (and correct!) info on temporarily importing your car into Mexico, then look no further. Not only do I keep up with the immigration and importation laws, I have personally done this twice.
The first thing I want to address is this rumor that if you temporarily import your car into Mexico, you must drive it back to the border every six months. This is false. I repeat:
YOU DO NOT HAVE TO DRIVE YOUR CAR BACK EVERY 6 MONTHS.
The most important thing to note about importing your car is that your car is legally allowed to remain in Mexico as long as your immigration status is valid. This means your FMT, FM2, FM3 must be current (not expired). You may renew your tourist or work/investors visa at any local immigration office.
The temporary vehicle import permit should cost you around $48 USD and you can get it at any Banjercito office (or online).
Make sure you have copies of your passport, your title and your insurance. You must have insurance that covers your car in Mexico. The car must also be in your name.
Any questions? Feel free to e-mail me! It really is an easy process and I've seen other sites make it sound waaaay too complicated.