I'm adding a factory sub to my 2005 ZX5. I do not have a harness back by the sub, but I do appear to have one up by the head unit.
Anyhow, I went over to Best Buy and picked up a harness adapter from the install shop and it included the sub adapter with RCA plugs! It is a Scosche FDK11B. Thought this might be of use to others.