I know this is an old post, but I see a TP1, TP4, and GND on my 404.
I tried doing a DISC TAP with TP1 and GND, and also TP4 and GND. I get nothing audio wise.
I even put a 10K resistor on the TP4 and nothing.
Any ideas of how you did it or pictures?'
Also, did you use a mono cable or stereo cable?