You probably just need to download the maps for Mexico. Garmins are really nice but they have a very sneaky planned obsolescence scheme. They typically put only enough memory in the device to hold the current maps, then over time they update the maps which you can buy and install on your Garmin - if you have the room. Otherwise you either have to manipulate the maps and select only parts of them so they'll fit, or you end up buying a new Garmin with slightly more memory and newer maps.
BTW, Kenwood makes in-dash radio/GPS units that use Garmin. I asked them if they had enough memory for future maps and they told me, "Yup, we have enough memory to hold any future maps Garmin releases." Yeah, and I've got a bridge I'd like to sell ....