The sacrificial device known as the fuse is the only protection your radio, lights, and other electrical components have. Technically speaking, a fuse doesn’t actually go bad -- a blown fuse has actually performed its job exactly as intended. That's a good thing, but it implies that there was something wrong elsewhere in the circuit. Due to the way fuses are engineered, the likelihood that a fuse would become faulty without blowing is pretty slim, but there are rare instances in which a fuse might appear completely fine, even though no current runs through it.
The most common types of automotive fuse are the blade fuse the mini fuse (though the mini is really just a smaller version of the blade). The two terminals (or blades) that go into each end of the fuse are only connected to each other in the middle by copper, silver, aluminum, zinc or some other alloy that can predictably maintain continuity for many years. The material inside the fuse must be able to handle its rated current capacity, and the center connecting element must respond to the slightest overdraw of current by melting rapidly.
The electrical circuits in your vehicle are designed to carry a very particular electrical load the battery to the components. The size of the wire and amperage rating of the fuse is determined by the current draw created by the connected electronics. Changing the components in the circuit can have an unpredictable effect on the amperage and resistance in the circuit, which is why you never jump to a higher fuse and never add extra electronics (like lights or a radio) that were not a part of the vehicle's original design. If there is a fault with one of your components or a short in the circuit, the extra current draw will likely exceed the amperage rating of the wire. This can cause the wire to melt and perhaps even catch fire.
Each fuse in your fuse box is designed to fail when the circuit is overloaded. The amount of time it takes for the fuse to blow varies depending on the material used in the center of the fuse and the amount of overload in the circuit. When there is excessive current, heat is generated and the center of the fuse melts and breaks the circuit to prevent further damage down the line. In many cases, you may be able to just replace the blown fuse and move on, because whatever caused the excessive draw was temporary or only created a momentary surge.
Remove the fuse that you suspect to be blown and visually inspect the center element through the case. The melting action often causes discoloration of the case, and the center element should have an obvious break. Another thing to check: Look for a small square opening on the top of the fuse above each blade. Set a multimeter to the continuity setting and probe these openings with your multimeter leads and see if there is continuity through the center portion of the fuse. If there is little-to-no continuity, the fuse is bad and needs to be replaced. This is a good test to double check a fuse that is not a part of the car's circuit, but do not test the fuse while it is in the fuse box. That's because if there is a serious problem with the circuit, the multimeter will complete the circuit, damaging your meter.
