That means when you turn the key to start tractor that wire get 12 v, thereby power the cut off relay (And cut power to all GPS related systems, when starting engine)
In my setup I tried to use as many of the original wires as possible, but not so easy!
But my Button is only to activate autosteer (steer input on PCB) One wire to the PCB, the other to same GND as the others, at the right side (behind side trim (to the back) about 40 cm below fuse-box)
I guess you have steer ready tractor!
Ignition must be on and the original Autosteer switch on tractor panel must be on, before the two relays which allows connection all the way from x716 to coils at valve (the 2 left/right coils), are activated.
Edit: as it seems you don´t have a correct wire sheet for your tractor, then disconnect x710, ignition on +autosteer on and measure connection where those wires go (or read number on wire and find other end, color may shift but numbers don´t.)
Today I recommend to make your own new (and thicker) wire for 12 V, instead of the thin 3187.
You should also leave the GND/return wires 3164 and 3166 in x710 and take them to your Cytron (or IBT_2) from X716
(Maybe on your tractor 3164 and 3166 already are put to GND somewhere else compared to mine. AKA not going to x716 anymore)
So only thing to find in x710 is the 3168 (on the side going to dump valve) and put your own wire from 3168 to your box, where you have a high level triggered relay (search for: arduino relay high level trigger) or a mosfet relay like on the new Kaupoimod pcb v4 (also in suport folder)
On kaupoi v4 you connect 3168 to SW out 12V The other end of Dump valve has GND already on the tractor.
I would recommend to use the Steer.png file in support folder (same both in support download for v4 and v5) it is much simpler and the one I follow.
Today (v5) you should connect pressure sensor to A 0 on Nano and tune bar/cut off value in AOG settings. That means you don´t need the transistor any more. (find the Analog IN on PCB)
This also means, that you do not connect anything to the “encoder” input, because that info goes in via A 0
The steer.png shows a step up converter (12 to 24 V) You do not need that one.