Daytime in S.A. is a weak XECPN, "La Poderosa," in Piedras Negras with some splatter from local KAHL on 1310.
At sunset KXYZ in Houston starts popping in with its multicultural/Christian programming.
At night the KAHL splatter is reduced, and I mostly hear XECPN, KXYZ, and news talker KWHN in Ft. Smith, AR. I've also logged a very weak KNCB in Vivian, LA, and XERJ in Mazatlán once each. The former was just a station ID and ad, and the latter gave a 107.5 ID and was playing regional Mexican music.