[COBALT]

Any 12v auto circuit works by a difference between full potential (12v) and zero potential (relative ground, or near-0v) with measurable resistance so current will flow. Auto circuits don't have true grounds - in other words they don't terminate to a ground source like you'd see with household wiring. Since there isn't a true ground there's generally only two wires for every circuit: a power and a neutral for relative ground. Your live circuits that provide 12v should be insulated from the cab, frame, engine, and sheetmetal, so any ground to any circuit in your truck is connected to these things to create the difference in potential between 12v and 0v.

To make a ground that you can rely on to have near-0v:

1. Find a reliable 12v circuit that doesn't have a short or bleeds current off to another source. Make sure that's powering your circuit.

2. Find bare metal on your cab, frame, engine, or sheetmetal of your fenders, bed, or wherever, and mount a steel, brass, or copper screw.

3. Tie your (-) side of your circuit to that screw, and your circuit should be properly grounded.

A fast way to test grounds is to get an automotive tester with a 12v bulb in the handle. It should have a wire hanging out with an alegator clip you can connect to a power source, and it should be pointed like an awl so you can touch various parts of your truck to see if the bulb lights up. If it does (and it lights up FULLY) you just found a ground.