When you say "1 set of stereo RCA connectors", are there actually three connectors colored yellow, red and white? If so, use these and disconnect the coax from the cable box output.
Next, get a good-quality two-way RF splitter and connect its input to the cable from the wall. One output goes to the cable box input and the other to the TiVo RF input. Rerun Guided Setup and tell it about the new connections.
Next, get a good-quality two-way RF splitter and connect its input to the cable from the wall. One output goes to the cable box input and the other to the TiVo RF input. Rerun Guided Setup and tell it about the new connections.