So I embarked on this journey as well, and I gotta admit, I have found it frustrating. Trying to find information on the right wiring for the system, it's a bear.
In my case, I found that on of the wires off the connector to the original faceplate for the radio (I have the 4.3" screen) did work perfectly with ignition.
(Maybe too perfect, it shuts off instantly when I turn off the vehicle, not the "1 minute delay" or "until I open the door delay")
As recommended, I purchased AX-FDSYNC-SWC 2015, although since I have no steering controls, I am using only the RCA Video jack off the harness with the rest plugged into the 4.3" display.
What I noticed is the harness doesn't send the video feed to the display at all. But the little head unit knows when I put it in reverse and provides and error indicating there is no video. I was actually surprised to find that the camera doesn't even seem to shut off. It seems there is always an active video feed when the vehicle is on (I tested by activating the reverse wire manually).
My question is, did any one successfully locate a functional wire for reverse in the radio bay? I noticed one person sourced from the interior fuse box. I am making an assumption, either one of the wires that run into that unit are providing the "reverse signal" and Metra (annoyingly) didn't account for it in their harness. OR - there is no wire for the reverse signal in that compartment, and the "signal" is instead logical rather (with the control modules) rather than physical.
I don't want to make any permanent alterations down by the fuse box, I have hacked up the factory radio cabling badly enough. So I may use a fuse tap and wire it up there. But if someone has figured our the magical key to this, I'd like to know!