Just because you found one with 4 volts, it doesn't mean that's the illumination. Does the voltage go up and down with the dimmer switch and turn off completely when the lights are turned off? When the dimmer is all the way bright, it should be close to 12 volts. The ground should be easy to figure out- on the circuit board, it will be the wide circuit traces that go to every component and a dead give away is if a capacitor has a - symbol; it's what that's soldered to. Once you figure that out on the board, use a continuity tester or ohmmeter to connect it to a pin on the connector. Usually it's a black wire but not always. Or you can use a tester to hook to the truck chassis (any grounding point) and test each pin until it shows 1/2 ohm or so (ohm meter) or continuity. If you have trouble with any of this, you probably shouldn't be trying to do the bulbs.
