Do you know are you the first who at last have made automatic fader control for toyota amplifier? Congrats!
My resistor values for AVC-LAN are 47k-680-75 instead of yours 43k-510-150. Nice solution, agree with you. Unfortunately enhanced midrange PIC16F1xxx has comparators with greater possible offset than midrange PIC16 has, that is why I use old series. Another drawback is a latch-up risk on pins (9,10) protected by comparatively low resistance.
Usage of 7805 for automotive applications is almost a common "feature" for many post-soviet in-garage-designers
. Why don't you use automotive grade regulators? (rhetorical question). There are plenty of them.
Can not agree with the choice of general purpose MCP6004 for A2. Again, there are plenty of more "audio" ICs. Also ceramic capacitors in audio path are not nice.
There is a ground loop between adapter and head-unit, shorted by line cable. Enough said.
How stable are the results of amplitude measurements? Is there any discomfort in hearing at the moments of Front/Rear switching?
Special thanks for adding history of project (mainXX.c). Is it a copy from version control system or a kind of such system itself?