“Three months after Los Angeles’ only country radio station, KZLA-FM (93.9) switched to a rhythmic pop format, leaving the nation’s largest market for country music with nowhere to tune on the radio dial, Tim McGraw, Faith Hill, Toby Keith and the rest of the twang gang are back on the air. But now they’re heard on the AM dial. In fact, when the country music broadcasts started recently on XSUR-AM (540) — transmitting from just across the border in Tijuana — the effect in L.A. was almost like tuning in on an old crystal set with the signal fading in and out.” On Friday, Los Angeles station KKGO-AM will go country, too.