Times Mirror Co. is getting into interactive cable television.Its Times Mirror Cable Television unit, the nation’s 11th largest, is buying an estimated 5% stake in Insight Telecast Inc., creator of an onscreen TV guide that can also program a VCR. The deal is just the most recent for Insight. In the last month, KBLCOM, the cable unit of Houston Industries, also snapped up 5.8%. The pair joins other investors, including Viacom Inc., Tribune Co., Spelling Entertainment Group, Sumitomo Corp. and the Public Broadcasting Service. With Times Mirror’s 1.2 million subscribers, Insight now boasts an estimated 3 million potential viewers. Times Mirror Cable prez Larry Wangberg will join Insight’s board.