Where can I rent a car in Southern California to take across the border?
There are companies at the border of Mexico that allow you to do so.Here is one:US Mexico Car Rental
Based upon information I’ve received, your options are: Avis, Enterprise, or Baja Rent-a-Car in San Diego. All three will require you to purchase Mexican insurance from them. –Mexican Insurance agencies will write policies on any rent-a-car that does not specifically state in the contract that the vehicle is not to go to Mxico. Most, but not all, of the agencies in San Diego have this stipulation. Most, but not all, of the agencies in Los Angeles do not. Something for you to think about.