Go to a company called Bajabound on the internet (or call them) to buy car insurance for Mexico -- by the day or year.

The internet site will allow to select from five insurance companies and your level of coverage -- from just liability to full coverage. Most of the companies are American or American-backed. Bajabound is a very good company in San Diego and they cover all of Mexico. I have used them for six years.

If you need help or more advice, send me a private message. I take groups of cars across the border twice a year and have had no problems.