Originally Posted by
El Cochinito
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.
Add Fox rental car to that list.