From San Diego, what I would do is just walk across the border and then rent the car there. If you are already going to rent it's just so much easier. Though I guess it could depend on the border crossing.

Yeah, I was hypothetically asking about renting a car from a company I, or a friend, owns...

You need a letter of permission from a rental car company to allow the rental into Mexico. Most rental car companies won’t issue it; the ones who do have a border arrangement with Mexico. If it has foreign plates, you’re paying duty if you’re permanent.

https://www.mexperience.com/bringing-foreign-plated-cars-int...