I'm part way through setting mine up in the ute, the battery is going under the tray behind the drivers seat, the positive cable will run to a 200amp circuit breaker (should be mounted close to the battery if one is use), then onto a keyed isolator which is mounted to the seat mount so I can reach it if needed. I also purchased a distribution block but haven't decided if I'll use that or just take the additional power supplies off the isolator. Power and earth then run to the starter motor.
I went with 1/0 gauge cable and mainly used audio gear. The grey and clear cables are the ones installed now, the red & black were damaged by a panel beater when the tunnel was welded up...

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