I recently got a new truck and the stock stereo doesn’t have an aux input. I mostly listen to audio books in the car and instead of buying a new stereo, I was able to add 3.5mm socket that is directly connected to the audio amp in the stereo.
This is what I started with. Its a stock GM Delco stereo.
Inside, I found that it had a saa7706h dsp chip (datasheet).
I soldered wires to the audio outputs of the dsp chip and connected those to a 3.5mm socket.
Then I put it back in the truck.
It works pretty well. I soldered the jack in after the volume controls, so the volume is adjustable on whatever device is connected. There is a little static in the output, but that is due to the actual radio static playing softly in the background. I may just disconnect the radio completely and just use the aux input. I never listen to the radio anyway.