Mexican food is not a niche category in San Bernardino. It is part of everyday eating in a city with deep Mexican-American roots, with choices ranging from long-established sit-down restaurants to small taquerias and food trucks.
Do not reduce it to tacos
Tacos are an obvious starting point, but look beyond them. Depending on the restaurant you may find enchiladas, chile relleno, tamales, menudo, pozole, burritos and regional specialties. Breakfast can be just as rewarding as dinner.
If you are unfamiliar with a place, I often prefer ordering one of its obvious house specialties rather than choosing the safest item on the menu.
History matters here
Mitla Cafe is the famous historical name and belongs on a Route 66 itinerary. But a useful food trip should also include ordinary neighborhood restaurants that do not appear in every travel article.
Current quality and hours change, so check recent local reviews once you are in town rather than creating an inflexible restaurant schedule months in advance. A busy parking lot around lunch can sometimes tell you more than elaborate branding.