Originally Posted by
maltasr
In addition, no major rental car company in San Diego will permit their cars to be driven into Mexico and they will be certain to remind you of this at check out. This is primarily due to insurance concerns.
Actually some car rental companies in San Diego will allow you to take their cars across the border with purchase, from them, of Mexican auto insurance (averages about US$25 per day additional charge).
Thrifty
Dollar
Avis
Hertz
I used Travelocity to check the Geographic Restrictions terms and conditions for rentals originating at the San Diego airport.