Done a map update lately? If so, you may want to check your nuvi and see if it’s ready to head south of the border. The screenshot above is from my nuvi 765T after the latest map update. From GpsPasSion:
I believe Mexico is now included in the units/mapsets which originally included Canada and is also likely dependent on available unit memory. I believe the addition of Mexico requires an additional 80-90MB. Those with 2GB nuvis including the 1xxx series (1690 included) will likely not have Mexico available after the 2011.10 map update since those units come with only about ~120MB of free memory.
To see if Mexico has been installed on your unit:
Where To? > Cities > Spell > "Mexico City" > Done. Now select "Mexico City, DF", then press the Map button.