A car fuse breaks, or "blows," when its amperage rating has been exceeded. Repeated breaks may indicate that the replacement fuses do not have the proper rating or that the fuse is connected to an overloaded circuit.
Be sure to check, first, that the fuse has the correct amperage rating for the circuit it is in. The vehicle's manual should contain a diagram with this information. Check what devices or lights are on that circuit, as it may be the case that a new amplifier pulls too much power, causing the fuse to blow. The more devices there are on one circuit, the heavier the amp load.
Each electric device, including headlights and radios, pulls a certain amount of amps from the electrical system. The number embossed on the fuse, such as 5, 10 or 15, indicates how many amps may be pulled through the fuse before it blows. Each wire in a car has a specific gauge or thickness. The amount and type of conductor used in a wire determines how much power it can handle before it overheats and melts. Electric devices also have these same limitations. They can only handle so much power before catastrophic failure and possibly a dangerous fire occur. Fuses protect both the wiring and the devices in the vehicle.