Oil pressure sender unit confusion

Most test lights have enough resistance to test an oil pressure gauge. Install the light inline between the wire(s) in question and a good ground. Turn the ignition switch to the ON position and see if the gauge moves a little. If it does, you've found the correct wire for the sender. If it doesn't, try one of the other wires in the same way. You can also test those other wires to see if it will turn on a warning light, but, I don't think they had that option in a '79 pickup. I know my '78 doesn't have a light option along with a gauge. It's usually one or the other.