Very near the Mexican Border.
You can be sure Poncho and Cisco were not involved. Someone with enough credentials to be invited to the shindig or a loyal employee (perhaps a mex in that case)
About 200 Miles S.E. of El Paso, 30 mi S of Marta. There is an airstrip a mile or two to the East across Hwy 67.
Click on the Google Maps link on post 13, above.