Voltage drop testing is the best way to find electrical faults, at least in my opinion. So what is voltage drop? First of all a couple basic things about electrical circuits. Circuit means circle, electricity must complete a loop in order to have electron flow. If a switch is open, or a break in the wire there is not a flow of electrons, and electron flow is needed to perform work. The pressure needed to get the electrons to flow is voltage. A volt meter reads the difference in pressure between the two leads, that is all it does. For example if you connect your volt meter to a fully charged 12 volt battery the difference in pressure between the positive and negative posts should be about 12.66 volts. But if you touch the leads to each other there will be no pressure difference between them so your reading will be 0 volts. The same if you hooked up your leads to each end of an isolated wire, 0 volts. There will be a slight drop in voltage though, because perfect conductors don't exist. So if we have some kind of restriction in our conductor, maybe a bad connection, a loose crimp, corrosion, or some broken strands in our wire, these restrictions will cause a drop in the pressure of the electrons flowing through the wire. And we can measure this loss of pressure with our volt meter or dmm. (digital multi meter) But the circuit we're working on has to be operating with current flowing in order to perform voltage drop tests. And the voltage reading you get will be proportional to how bad the connection is, or corrosion, or broken wires, or whatever the problem is causing the unwanted drop in voltage. When a load, say a light bulb is in the circuit we should have source voltage coming into the load, in this case 12.6 volts. The positive lead on the dmm would go to the point of highest voltage in this simple circuit. That would be the positive battery post, NOT the battery connector or clamp but the post itself. Then place the negative lead on the dmm onto the positive wire or input of the bulb. This will check the entire positive side of the circuit from the battery to the bulb. There will be a slight drop in voltage compared to our 12.66 across the posts. There is a very slight voltage drop at every connection and switch in the path, and the longer the wire, the more voltage drop there will be. Say we had a bad switch on the positive side of our circuit, it would drop some of our voltage, say 2 volts. Our bulb would still light but it would be dimmer than it should be. So you can work your way back to isolate the voltage drop, You can even put the positive and negative leads on the in and out terminals of the switch. If you have 12.5 volts or so going into the switch and only 10.5 coming out, you have identified the problem, the switch could be bad, or the connections could be loose or corroded. If you test between the input and output sides of the switch, there should be 0.0 or just slightly more difference in the reading. If it is much more, the switch or connections are bad. But you must always check the negative side of the circuit also, it is just as important as the positive side. When the circuit is working properly, the load uses all the voltage. (Or almost all, there will still be a small amount) So if our simple light bulb circuit was working properly, we could check from the positive post of the battery. (I say the post and not the connector or battery terminal clamp because this will test the connection of the terminal to the post as well) to the input or positive side of the bulb. Generally the voltage drop should be 0.1 volt or less. Then we need to check the negative side of the circuit as well. Place the positive lead of the dmm on the negative terminal of the bulb, and the negative lead to the negative battery post on the battery. If the reading on the dmm is not 0 or 0.1 or very close to it you have a problem on the negative side, possibly a poor connection or corrosion is causing the unwanted resistance. When testing I like to run a ground wire directly back to the negative post of the battery, this will eliminate any chance of grounding your dmm to a poor ground and getting faulty readings. Some people prefer to disconnect wires and test using the ohm setting on their dmm, you can do this but say you have a wire with corrosion on the inside of the insulation, or all but a couple strands of wire are broken. You would still get a good reading with an ohm meter continuity test, but the circuit would not operate correctly under a load. Or if you take out a dim bulb and check the voltage on the positive side of the bulb, you will get a good reading of 12.6 volts on our circuit but it won't light up nice and bright with the bulb in. But you would assume the wire to the bulb socket to be good when it is not. Very often corrosion is hidden inside the wire's insulation and you don't see it by looking, this corrosion reduces the wires ability to carry the amount of electron flow needed. Some guys poke holes in the wire insulation when testing, I never do this, because that is just inviting corrosion into the wire. But if you insist on doing this, at least seal the hole you made with some of your wife's fingernail polish or something. Don't use silicone though it creates ammonia which causes corrosion. Again I think the voltage drop test is the best way to find electrical problems. If I'm helping a friend or someone with his jeep or car even if the problem is not electrical I do a couple basic tests, like checking engine and body grounds. Connect the dmm between the negative post of the battery and the engine , and with the fuel pump disabled crank the engine a few turns and check the volt reading it should not be more than 0.5 volts at the most, lower numbers are better. Then check between the negative battery post and the firewall with the engine running and all the electrical loads turned on, again the lower number the better. If anyone is interested I will post some other tests to perform.