first, stock 2600's just output RF, so forget about the AUX/Line in's. Check what the 2600 is switched to.. more then likely channel 3. Then change your tv to channel 3.. if the magic doesn't happen now, it never will. Go get one of these
It's just a RCA to Coax adapter. You can get one at Radio Shack for $1.99. Plug it up to the TV Cable TV input and if that doesn't work nothing will.