Are you referring to my circuit in post #22 ?
Using the diodes to combine two outputs did not work, that is why they are scored out in the schematic sketch.
There have been other discussions about my circuit, have a look here:
https://www.electro-tech-online.com...ols-to-aftermarket-radio.153039/#post-1317989
where you will find the final version of the circuit.
JimB