This article is from the Fishing bass FAQ, by Bass Rogue with numerous contributions by others.
The most common tool for troubleshooting electrical problems is a
voltmeter, a device use to indicate the presence of a voltage.
Personally, I think this is a big mistake when chasing difficult
electrical problems. The impedance of these devices is so high, you
can't load down a faulty connection with them. In other words, you
can read 12 volts (really 13.2 volts) on the meter, but you can't pull
enough current through a faulty connection to light a bulb. I prefer
to use a bulb with two test leads (wires) attached to it. If the bulb
comes on when I connect it, I have voltage and current, and current is
what makes a bulb glow.
Besides the test bulb or a device to check voltage, the only other
special tools are things needed to make reliable electrical
connections. These include a crimping tool, wire stripper, wire
cutters, fine sandpaper (emery cloth) and dielectric grease. The
thing to remember about electrical connects is they must be clean,
mechanical sound and protected. A clean connection means the wires
are shiny and free of contaminants before you connect them. A
mechanically sound connection is one that is made by crimping or by
forcing the conductors (metals) together. Once you make a good
connection, protect it. Use dielectric (non-conducting) grease to
seal crimps or connections. If you can't find something called
dielectric grease, get an automotive silicon grease (clear).