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The "switched" wire is goes to something that will turn on with your engine and turn off with your engine. So the ignition wire is where you'd splice it into. The other power is a constant power wire, so look for a wire that has constant 12V source at all times. One of the grounds you are talking about is probably a power wire for the backlight of the gauge. Find a wire that turns on with your headlights or running lights. If there is no backlight, then simply ground the wires it tells you to.
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