Cox Enterprises Inc. said Monday it has made a tender offer for all the outstand ing shares of Cox Radio Inc. (CXR) for about $69.1 million. The offer for $3.80 a share, represents a 15% premium over Friday’s closing price. Cox Enterprises owns about a 78% stake in Cox Radio. If that figures reaches 90%, Cox Radio would become a wholly owned subsidiary of Cox Enterprises. The company’s operations include cable-television provider Cox Communications and television-stations and newspaper publi