I only need aux input but as far as I know, it cannot be done without this. If I had a CD changer, then I could just insert some CD into that and connect my own audio source into those line level signal wires. But without that, those inputs behind the radio unit cannot be activated. True AUX input works only for late 2002 or 2003 model year or something like that but mine is a bit older.
Ps. Glad that you found the issue for your problem. I'll have to keep that solution in my mind too.