electrical gremlins are back

one of my buddies has that power probe so I am going to ask if I can borrow it. But as of now, I get 5v at the red and black 10 gauge wires at the splice in the cab with the key OFF. when I turn the key ON, it drops to .1v. And when I either open the door, or pull out the headlight switch, the voltmeter reads 10v.