Protestants are almost a plurality in Guatemala and Honduras? Now I knew that the Evangelicals were big there, but....
Interesting that the Protestant population is still so low for Mexico in comparison.
Mexico is uneven. In Chiapas the Catholics are, I believe, under 60% by now. In Guanajuato they are well over 90%. So, it is not that the border is sharp. Then, again, evangelicals are the biggest among the native and the poor. The wealthy and educated classes tend to stay Catholic (if nominally).
In northern Mexico and parts of central Mexico there are non-negligible numbers of Protestants that converted in the late 19th and early 20th century. This demographic is very middle class and was liberal/revolutionary in the early 20th century but now tends to support the PAN.
In response to Snowstalker:
Presbyterianism is the most prominent Protestant denomination in Mexico, even in Chiapas. Many "mainstream" Protestant churches have a very significant presence throughout Central America. These Protestant churches tend to win converts based on their message of Christian univeralism and their superior provision of education/various social services. I can't say that the same is true for Evangelical churches in Central America but this is hardly a manifestation of the "far-right". If anything, the spiritual message of Protestantism succeeds in Central America because of its spiritual egalitarianism and is reflection of leftist values/aspirations.