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Just a word of warning about those test lights. If you have *any* air circuits (small plastic tubes for air that look like really nice thick wires), do NOT puncture them. I offer this advice because I just barely prevented someone from doing that to my fast idle control o my bus.
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You really shouldn't puncture ANY wire insulation with test leads, especially in a vehicle. All it takes is a pinhole, and condensation will get in and turn your wires to green powder. Sometimes you can't even find the hole, but when you feel along the wire you'll find a place where it is stiff and resists bending, cut it open and there's your powder. Even worse, the powder will show 12 volts through the wire when there is no load.
Do yourself a favor, and get in the habit of measuring at the terminals. Make sure your mechanics do, too.
If the power in the wire seems good, do as The Experience says and measure across the load with the power on. If the voltage is low, measure battery to hot, or hot to the frame, and ground to the frame. If you have voltage between battery hot and load hot, or if load hot to frame is much less than battery hot to frame, then there is resistance in the hot side. If you have voltage between the load ground and the frame, there is a high resistance (bad) ground.
And yes, I agree the handle switch starts and stops the red lights if armed at the beginning of a run. When I was in school, there were no yellow "Jersey" lights to trigger, but there were door handle switches on the reds.