This is an update to Notes on Installing Sony XAV-AX100 in a 2010 Prius, specifically trying to integrate the factory microphone.
Unfortunately, the microphone is pre-amplified with what I believe to be a New Japan Radio Co. 2140 Op-Amp (the black IC in the middle on front side, pin 1 is upper left corner).
When connected to the factory head unit, the output signal of the factory microphone module was about 500mv peak-to-peak and centered on 0.0v.
I wasn’t able to get a great oscilloscope capture of the factory microphone output when connected to the Sony head unit, but it ended up being biased by about 3.3v and about 400mv peak-to-peak.
I could have re-wired the factory microphone circuit to remove the amplification, or even made a new circuit to de-amplify it and put that inline. But I decided that was too much work and just installed an external 3.5mm microphone instead:0