CNN has equaled and perhaps surpassed the three commercial networks as the nation’s most believable TV news source, according to a new survey that suggests NBC’s credibility was hurt by its staging of the General Motors truck explosion.The new poll also indicates people don’t believe their daily newspapers — or the Associated Press — as much as they used to. They apparently believe politicians even less, and Geraldo Rivera hardly at all. The national survey by the Times Mirror Center for the People & the Press, released yesterday, found that the Cable News Network had eclipsed NBC as the most “believable” network news operation in the four years since a similar survey was conducted. In the new survey of 2,001 adults conducted Feb. 12-21, CNN was closely followed by ABC and CBS, with NBC fourth. All but NBC were within the poll’s 2 percentage point margin of sampling error.