The Short Version
No evidence supports the existence of this survey or its reported results. None of the credible audience measurement sources (Nielsen, Pew, Edison Research) reference a "DWSA radio" survey, and the only entity matching "DWSA" in the evidence appears to be a water/sewer authority, not a radio station. The specific 18/20 split from 38 respondents cannot be traced to any primary source, dataset, or publication.