Electrical problem
1988 Ford Ranger 4 cyl Two Wheel Drive Automatic
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I cannot determine which cable is positive or negative. One of them go to a thing connected to the firewall on the passenger side of the truck and the other has three wires coming out of it. One goes to a bolt on the front of the truck body , and another goes under the motorsomewhere. How do I tell whcih one is which?

The one with the 3 wires 2 black and 1 black and light green wire goes to the negative terminal/post of the battery-