2016 FR-S Head Unit stock connections
 

I'm trying to map all the IO for adapting to aftermarket head units. I'm installing a Pioneer, but depending on how the Kenwood units due in march perform, I'll install one of those. If I can, I want to avoid splicing into the factory wiring.
Anyway, I started a spreadsheet (I'm an engineer...), but forums don't like tables, so below is a png of what I've been able to dig up so far. Please let me know if you know what any questioned signals are, or see errors. The perspective is facing into the existing wiring, or looking at the back of the new connectors / away from the radio, which is not quite canonical, but should make it easier to wire up connectors.

I got some good feedback via PM, and did some scoping of the mic signals, so the table has been updated.

The mic signal on an AVH-X5700BHS appeared to be unamplified condenser signal with a DC bias of ~1.4V, which only came on when the microphone would be used. The audio signal was low, maybe .1Vpp for loud noises.

Any clue how to get the stock backup camera working on an aftermarket HU?

There are 2 parts: the wires, and changing the aftermarket Reverse voltage out (usually 12V) to 6V.
You can buy a harness with the voltage regulator built in e.g. http://www.autoharnesshouse.com/11443.html

Or you can build one yourself, but be careful not to fry the camera. It's a good idea to use automotive rated components (the regulator is the main concern here), since temperatures can get hotter & colder than general electronics do.