Gauge Installation on a Gen 2 Dodge Cummins

All gauges need to get powered when the ignition key is on. To find a switched ignition (keyed hot) source, use your test light or Voltmeter and check the wire you have decided to use. Ground the test light or voltmeter to the metal chassis and check the wire with your test lead. You should have 12V+ with the key in the ON position, and no voltage with the key in the OFF position. 
On this 2002 RAM the wire pictured is the Black wire w/ Brown trace below the main wiring block under the dash.  Install a wire tap on the wire.  Notice the in line fues assy. with a 5Amp fuse.  Here the 3 red gauge power wires are connected to a yellow male spade terminal.  The Yellow male spade term. is plugged into a blue female spade term. and then to the fuse assy. From there it is connected to the trucks ign. hot wire with a blue male spade terminal that is plugged directly into the red wire tap.