Do they make a RCA cable that has RCA connections on one end of the cable and the other end so it is pos and neg to go on a speaker terminal? Or an adapter than can accomplish this?
Are you trying to power speaker with RCA output? If you are then you should know the only adapter would be an amplifier. In a bind I've soldered RCA ends to speaker wire to cover the distance to the amp, but if I tried to connect a channel directly to a speaker it wouldn't have worked. I'm sure some can explain it more technically, but I'll just say speaker output is considerably stronger than signal output.