I would pickup a simple voltage detector from your local hardware store. Very simple to use, the type I'm referring to, you just hold the probe on the insulation of the wire and a light comes on if voltage is present. Test the wires, making sure the switch is on, and the black wire on your new light goes to the wire with voltage. Make sure the switch is off before wiring up.
P.S. make sure the tester will detect 12 volts.
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2001 Dodge Ram 1500 Reg Cab 4x4 OFF-ROAD