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Understand how bad amplifiers can affect sound. Open the fuse box. Get out your multimeter and set it to the conductivity test. Hook up the multimeter to the fuse box. Listen for any beeps. Turn on the car, and test the speakers.
If there is something wrong with the amplifier, you will likely hear some sound distortion when you turn on your speakers, or nothing at all. This is usually because there is something wrong with the fuse or the capacitor. If you don't know where the fuse box is, you can check online or in the manual that came with your car, as each car will be a little different. A fuse box will usually be located in the front of the knee-well or under the dashboard, however. This will help you figure out whether or not the fuse is good or needs Touch the multimeter's red wire to one of the poles on the fuse. Touch the meter's black wire to the other pole. If you hear a beep, then the fuse is good, and your problem is likely with the capacitor. If you don't hear a beep, then the fuse is blown and needs to be replaced. Be sure to get the exact same fuse model. If you hear a beep, consider replacing the amp first. They are usually less expensive, and don't require soldering irons and desoldering pumps like new capacitors do. They should be working now. If they don't, then there might be something else wrong with your car's speakers. Consider taking your car in to the repair shop, and having a professional take a look at it.