wiring harness testing

Just about any power source that can operate a stop lamp will do. Car battery, bike battery, what have you. See if you have an old wall wort. Read the label. Look for something 12-14V about 1 amp or heavier output.

If you use a heavier power source, wire a stop lamp in series with the power, so that if you short something, the lamp will act as a protector and not burn anything up, plus the lamp will light up and indicate that you have zero ohms continuity or a short

There are lots of ways to do this. Your multimeter will show which wires have continuity but you cannot load test connections with it.