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You don't want constant power for gauges, it needs to be switched with the key unless you want the battery to be dead all the time. I run mine off the stereo circuit -- hot in accessory and run.
I have those same gauges. I used the radio circuit and the needles return to zero. I used a test light to see which wire was switched 12v and which was constant 12v.
Any chance you recall if the radio plug had both ?
It does.
cant remember colours, but there's the switched power which lets it turn on, and a constant 12v which is why it doesnt lose your radio presets etc every time the car is off.