THE NEXT TIME you turn your back on a television commercial, remember this: You may be walking out on another Petula Clark.
      For Petula, the "Downtown" girl who does her second NBC special Monday night, admits that if it weren't for those auto commercials she does, she might never be doing a special.
      Five years ago, so far as most Americans were concerned, Petula Clark was a voice apparently built into all radio sets, and the voice sang of "Downtown." Then the auto commercials gave the voice a body and Petula (a combination of Pet and Lula) was on her way.
      At 34, the English singer-actress and her manager-husband, Claude Wolff, can be very selective about the things she does.
      Thus, in her internationally-flavored special, tiny Petula will be joined by England's Oscar-nominee Ron Moody, the Fagin of the "Oliver!" movie; France's guitarist-vocalist Sacha Distel, and Andy Williams.
      It is Williams who best describes her appeal:
      "She sings young songs in a young style, and still adults can

identify with her as a human being because she has maturity."
      Petula, her husband and their two children (Barbara, 7, and Catherine, 6) have a big, permanent home outside Geneva, Switzerland. They also mantain residences in the United States, England and France.
      That takes plenty of money in any language, but Petula is singing for far more than her supper these days.
      She received $200,000 for starring with Fred Astaire in the "Finian's Rainbow" film, and double that figure for doing a movie musical adaptation of Mr. Chips" with Peter "Goodbye, O'Toole.
      The way this girl is moving up, pretty soon she'll own all of "Downtown."
      She has, in fact, been on her way since she was a seven-year-old child in London.
      "I had a bit part in a BBC radio show beamed to the troops overseas," she recalled. "An air raid started. To quiet the kids in the studio, I was asked to sing. I sang `Mighty Like a Rose', one of the few songs I knew."
      BBC officials were so impressed, they launched her as a star by giving Petula her own show.