yes the pre-wired harness is there, it's just plugged in already so it's confusing. You have to unplug it and put the new harness in line then plug the whole thing back in from where the original came out.
I went to cdchangersonline.com and bought the changer for my '99 se wagon and my '01 sable also, it comes with everything you need and it takes no time to hook up if you follow the instructions. It's 2 or 3 hundred dollars but well worth it considering what I would have gone through otherwise.