Solved.
I came across this thread when troubleshooting my AVH-1300nex after installing my backup camera.
Initially I spliced the camera power into the reverse light power, as well as the ground cables. I used the AV cable that came with the unit which had an additional red line. I hooked the red line to the reverse light and the other end into the head units reverse signal input line, and the RCA plug into the RCin port. The result was the head unit displayed the camera all times EXCEPT when I reversed. I ended up having to wire up a relay that disconnected power when in reverse and had power enabled in all other gears with the relay wiring as such:
85 - ground
86 - red line connected to reverse light
87 - unconnected and protected from grounding out against anything
87a - H/U power cable
30 - reverse signal input
This worked for me, hopefully it helps someone else, ask if you have any questions.