My heater is giving me problems, and I want to confirm that the wiring between the control panel in the cab and the heater is good. I couldn’t figure out how to do this, because my multimeter’s leads are a few feet long at best. Here’s what a quick Googlin’ turned up:

This is easy to do without the need for extra jumpers if you can isolate both ends of the wire you want to test. You can either do it by voltmeter or testlight, or ohmmeter:

Just clip your testlight or voltmeter to a power source on one end (alternator positive pole is always hot in the engine bay), the other lead to the wire you want to test. If the wire is isolated from ground, you would see no voltage or the testlight will not light up. Then go to the other end of the wire and ground it, your testlight will glow, or your voltmeter show voltage.

Or use the ohmmeter, connect the leads between any grounded point and one end of the wire you want to test, there should be infinite ohms, then ground the opposite end and you will see a circuit. This method is easiest because you don’t need to find a positive power point. Best of all is if your VOM has a continuity buzzer you can put it in ring mode and test this way without the need to see the meter.

Thanks to user tencentlife on thesamba.com (thread is here).