I have always hated those adapters, something about a wire hanging out of the tape player just makes me squirm.
I did spend some time messing with it. I found the left and right outputs from the tape player and spliced in a headphone jack. I could get the audio to come through by "playing" a tape, but for some reason it kept spitting the tape out after only 5 seconds or so. I put a multi meter on each wire in the harness to see if there was any change in voltage when putting a tape in. Found nothing.
I ended up ordering a PAC adapter that uses the tape plug in the back of the head unit to add two sets of stereo inputs. This should also let me plug in the overhead DVD player when I get around to replacing the broken one I have now.