XETRA

XETRA "91X" has been an Alternative Rock station in San Diego, California since 1983.  They are licensed across the border to Tijuana, Mexico.  XETRA airs the Mexican national anthem twice a day as well as La Hora Nacional, a one hour program that airs on every Mexican radio station at 10 PM on Sundays.

This chance that this license plate is referring to XETRA is pretty slim but my radio nerd mind would like to think it is.