What I have also done is make up a bunch of short test leads with different connector ends. This allows me to test circuits at a harness connector with out risk of damage to the connector. I have a variety of male and female connectors in spade and round types that I have accumulated over the years.
Another tool you will need is a soldering iron or soldering gun. I use both but for the average person a gun would be more useful. Use a low wattage gun. I have a 40-watt soldering iron and a 100-watt gun for small gauge wire and a 180-watt gun for larger wires. Anything bigger will be too hot and burn up the wire before you can get solder on it.
You'll need some thin 60-40-rosin core solder and some soldering flux. Fine emery cloth or sandpaper and a small wire brush for cleaning wires and connectors. Never use acid core solder on electrical applications. It will eat away at the wires in short order creating a problem down the road.
There are other, more specialized tools for troubleshooting electrical problems, such as scan tools, logic probes and self-powered test lights. For the average person working at home, these basic tools will do for the most common electrical problems.
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