Or, you can add a switch and diode to do the hack electrically:viewtopic.php?f=36&t=3815&start=10
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In my Prius I have a SPDT center-off switch that does both the NAV hack and the backup camera hack on the same switch. It works 100% the same way on a LEAF.
I did this backup camera hack on my LEAF today. It is such a useless but funny mod
You are right you only need to override the reverse signal wire---I used the service manual from NicoClub http://nicoclub.com/manuals/Leaf/2014%20Leaf/av.pdf
to find it. Even though the manual is for 2014, the pinout and wire color on my 2017 non-Bose navigation matched exactly (45, green).