It isn't set in the Yucatan, it's in the Mexico city area, but this could be Clash of the Star Kings by Avram Davidson. This was published in 1966 so you could have read it 55 years ago. It was nominated for a Nebula so it was quite a well known book at the time.
The Aztec gods turn out to be aliens who arrived in the distant past. The book is set in contemporary times (i.e. in 1966) and the plot is that one faction of the aliens now want to take over the whole Earth:
On one side, the Great Old Ones, who had benevolently ruled the land in the days of the Toltecs, before the coming of the cruel Aztecs.
On the other side, the Huitzili, who had come from the distant Evil Stars to teach the Aztecs conquest and war. In return, they demanded endless human sacrifice, human blood, and human hearts. And now, if they gain control of the awesome secret power within the mountain, they will plunge the entire Earth into endless barbarism!
Quetzalcoatl is mentioned, though the story isn't specifically about that god.