FUN FACT: If you abandon your foreign-plated (California) car in Mexico, you’ll have to pay Mexican Customs 40% tax on the car’s value. This rule was established to discourage foreigners from abandoning their California vehicles in Mexico. There is an option of donating your vehicle to Mexican Customs if you want to dispose of it legally.
Once you've completed the necessary transaction in Mexico required to remove yourself of the vehicle's ownership, submit to the California Department of Motor Vehicles a "Notice of Transfer and Release of Liability". This will tell the DMV that you no longer own the vehicle. The Release of Liability form should include the buyers name and address. You may complete the process online here.