The 'border blasters' started operating in the '30s, though not at the power that they did later.
I remember listening to one from Del Rio, TX when I was a kid in NE Illinois in the '40s.
Incidently, singer Slim Whitman got his start on the 'border blasters'.
I get a little magazine about old-time radio called 'Nostalgia Digest'.
The spring 2003 issue had an article about the 'border blasters'.
My recollection is a personal recollection of such stations coming out of Del Rio as XERF ... growing up in the Chicago - Grand Rapids - Detriot triangle we didn't hear a lot of other stations from the southwest part of the world - but I remember the 'corn' coming out of Del Rio!