What do I need to cross the border?
You need the current registration for your car. It should be in your name( not a lease or rental).If the car is not in your name then you will need a letter from the owner of the car stating that they are giving you permition to take the car into Mexico.
What does Mexican Law say about foreign visitors?
The Government of Mexico requires that all U.S. citizens present proof of citizenship and photo identification for entry into Mexico. However, U.S. citizens have encountered difficulty in boarding flights in Mexico without a passport.