Yes, there was an assembly plant in Mexico. And, get this, the mexican government would not allow Fastbacks to be built!!
This is why the 69 "Shelby de Mexico" has those long roof flairings, to make it look like a fastback
Yes there was an assembly plant there... as mentioned only coupes were built.
A few "Shelby" coupes were assemblied by a dealer down there (early ones had no real body modifications like the 69's) under an agreement with Shelby. And after the Shelbys ended here, they continued to build "Shelby" Mustangs, Mavericks and Galaxies for a year or two.
A fair amount of the Mexican coupes have found there way north in the last ten years.... more as oddities and something out of the nor at shows