In addition to the L+R 'audio in', I think there's a data link that the head unit uses to communicate with the CD autochanger. It uses this link to change CD's, skip tracks etc and indeed see if there are any CDs in there at all. Unless the lead you bought has some clever stuff in it to fool the head unit into thinking it's talking to the changer I'm not sure how it would work.
Brian's suggestion should work though, if you modify your existing lead to patch the 3.5mm socket into the audio L+R channels (into the head unit). However, if the audio lines from the CD autochanger have been cut I'm not sure why you'd need a CD of silence - you can fill the CD cassette with your old Perry Como and Osmonds albums, safe in the knowledge that you'll never hear them. I've not tried this by the way, and I'm not sure what will happen when it changes the CD's - at worst the head unit might mute the audio but otherwise it should just keep playing and changing.