Interfacing the OEM radio/nav to the 5.1 sound is the new project.
Tried using LOC's and the distortion from the transformers is too much to bare.
Going to use a differential to single end conversion chip (4, dual amps on one IC) to convert the differential signals (pre amplifier) from the OEM and mix them then feed to the 5.1 decoder.
We shall see.
I had to get two switching power supplies that put out 15 volts (adjustable) so I could supply the positive and negative rails for the chip to work.
Then I found software that actually made it play through the Mac Mini.
"Linein", "Soundflower", and "AU Lab".
So now I have volume and tone control over the OEM audio feed.
Here is the interface.
2 ICs, 4 caps, 6 resistors.