hack aux onto pioneer ke 1800qr

I can only imagine that tapping into the head wires has some serious noise issues.  you should be able to find a better place to connect it.  You will need an amplifies computer speaker that you can sacrifice the input cable.  Cut the end off of it and strip the wires.  There will be an outer shield and 2 center conducots.  Attach the shield of the cable to the chassis of the radio.  Now wih a tape playing in the mechanism, touch one of the center conductors to each contact of the connector that attaches the cassette mechanism to the mainboard of the radio.  Hopefully 2 of the connections will have audio.  You wil need to somehow isolate that connection from the cassette mechanism.  Either cut a trace or remove a component.  Istall a 10 microfarad non-polarized capacitor on the connection that goes to the mainboard.  One cap on each of the 2 that had audio.  Other end of the cap to the center conductor of your aux cable.  Ground the shield of your new aux in cable to the tuner pack of the radio.  It is the easiest place to solder this wire to.

If there are 2 output chips in that radio, chances are there is a 7280 and a 7281, check carefully to make sure.  Those ICs are capable of several configurations.  You will need to use the computer speaker to tell me which pins of the chip have LOW LEVEL audio getting to them.  On the 7280, check pins 1, 2. 4 and 5.  There wil be audio on 2 of them.  I need to know which 2 they are.

7281 check pins  8, 9, 11 and 12.   which 2 of them have audio.