I think it's fair to say that San Diego is a leader in terms of the availability of 24hr Mexican taquerias.
And it's not hard for me to nominate La Posta, on Washington and 3rd St, as overall king of them all.
If you're looking for dining options after 10pm, you're likely going to have to choose a 24hr place. I go to La Posta anytime, actually. it's that's good!
Call an order into La Posta and you won't be sorry. Their Salsa Verde is amazing btw.
The seating is only outdoor seating, so you may to order "to go"?