Aux Input Replacing Cassette

[autotech44]

Hey,

I'm looking to add a phone aux input to the OEM Pioneer radio/cassette unit in my '91 LS400. I read "Hardwired An Aux Input To '91 Pioneer LS400 System" from about 10 years ago but there's something missing.

I'm trying to do it right without using a cassette adapter. I figured out how to modify the switches on the cassette mechanism to fool the controller into thinking a tape is playing, so once the aux is installed I can simply press the tape button to switch to aux (phone).

I tried disconnecting the head and soldering the 3 aux wires in there. It sounds like hell I'm assuming because the aux has to be installed after the pre-amp? It sounds bad, but works great as far a AM/ FM functioning and switching to phone using the Tape button.

I found 3 test terminals that are after the cassette pre amp and installed the Aux there. Sounds good, but now the problem is the phone audio is there using AM/FM or tape, and it kind of overrides the AM FM audio as you tur the volume up on the phone.

OK. Read this article and it seems like this gut knows how to do this correctly. https://hackaday.com/2021/07/28/how-to-modify-your-car-stereo-for-bluetooth-or-aux-in/#comments

If you read Guide 2 you'll see him discuss a chip called a mux or multiplexer and how this is most likely the cause of the Aux input going to AM FM and Tape as opposed to Tape alone when the button is pressed. This method should also allow me to completely remove the cassette mechanism since I no longer need to fool the controller. 

Anyone been through something similar? Again, it will be so cool if I can make it work by simple switch to Aux when I press the Tape button. 

I have the service manual and schematics on the unit, just need a little help finding and adjusting the MUX???