There's a lot of non-corn Mexican food. A buritto for example has a flour tortilla, beans, rice, cheese, and maybe meat and sala in it... no corn. Just avoid things with corn tortillas (or ask for flour instead) and certain types of salsas and you should be okay.

Just tell them you are alergic to corn (even if that's not the issue) so that they will make sure to let you know if that's going to be invovled in what you are eating, or just ask for a detailed ingredients list.