Electrical problems may seems intractable and confusing, but they're actually fairly easy to diagnose once you know the basics. An electrical system can only fail in so many different ways; once you figure out what kind of failure you're dealing with, finding the source is just a matter of time and persistence. Just remember that electrical systems aren't week-old hot dogs. They don't "go bad," they just malfunction in one place or another.
Electrical system problems will often initially manifest as blown fuses. Fuses are the weak link in your electrical system, designed to overheat and fail before any of the accompanying wires do. If the bare metal under the insulation of a positive wire touches the frame or touches the bare metal core of a ground wire, the heat created during that brief contact will effectively weld the two together. The result is an overload in the circuit and blown fuses. Bear in mind, though, that frozen or malfunctioning electric motors can also cause blown fuses.
The average electrical system is a study in engineering convenience on the part of manufacturers. A car maker will often connect completely unrelated systems through the same electrical circuit; radios run through the dome light, transmission controllers run through the gauges and engine controls share a circuit with that electric flashing skull shifter knob. If activating your windshield wipers turns your radio off or rolling up the windows kills your engine, then odds are that you have an unintended crossover or short within a single system.
Resistance in a circuit causes electrical energy to get stuck at the point of resistance. Since the energy can't go forward and more energy keeps piling up behind it, the electrical energy begins to "vibrate" and create heat. Since a given conductor can only move a certain amount of energy, any heat in the line will create a cascade effect of heat buildup. Eventually, the heat will get high enough that the circuit will blow a fuse or fail to transfer power to the accessory.
If you park your car at night and constantly find that the battery has died by morning, then odds are that you're experiencing some kind of short or amp draw (accessory not turning off). To test for a short in the system, remove both battery cables and connect a digital multimeter (set to ohms of resistance) and touch the probes to your cable terminals. It should read somewhere around 100 ohms; if it reads zero ohms, then you have a dead short in the system. To test for amp draw, disconnect the positive terminal, set your DMM to read in the 10-amp range and touch the probes to the positive battery terminal and battery cable terminal. You should get a reading in the milliamp (0.001-amp to 0.009-amp) range. Any higher than that and something isn't turning off.
