A while ago, Alex (kare_raisu) posted a review of a mariscos truck doing what he described as the best Baja-style fish tacos in SD. I've now been a couple of times, and can report that he wasn't kidding - these are some great fish tacos. Nicely fried, on fresh tortillas, with just the right amount of cabbage and white sauce. The salsas on hand are pretty good too, one habanero, and one chile de arbol (I think). They also have raw red onions w/ chilies and black pepper, which add a nice bite to the tacos.
The name of the truck is Mariscos El Pescador. Here's a map to the location:
Definitely worth your time. Fish tacos are only $1.25. I can't think of a better lunch for less than $4.