One of the maps on Google (
http://www.traveljournals.net/explore/mexico/map/p6241670/ma...) shows Maritas and also tells you the latitude and longitude settings for it. Armed with that information, you should probably be able to pinpoint it into your travel plans. When you are looking at the map, click on the pushpin that indicates Maritas and it will popup with the location information.
But I have a question for you: Can you not write to the Registro Civil for that town or the Archivo General de la Nacion? They should have that record for you, depending on the year of your mother's birth.