I looked into this last time I had my deck apart to fix the volume knob. I could not find any datasheets to the cassette IC, and the signal has to run through the decks other IC's, for use with the volume, fader, bass, treble, etc., before hitting the power amp. I usually don't find it difficult to follow traces and discern which are the audio signal traces, but in cassette IC's case I was stumped. Hopefully, I can accomplish this in the future as well because mine won't play tapes anymore, anyway. You would also have to accomplish manually enabling the cassette portion so as to use it's audio chain... either that or do it the ghetto way and leave the cassette mechanism intact and use a dummy tape so it engages it's switch.
Note: Actually, now that I think about it, I wasn't examining the cassette IC, I was considering bypassing the radio portion since I don't normally listen to the radio. GAH, now I wanna open it back up. >_<