The job of an inline fuse holder is to protect your vehicle in case of an accident. Should the car audio system’s main power wire have a short to ground, the fuse will blow and prevent additional damage from occurring.

Typically you want to mount the inline fuse holder as close to the battery as possible. General rule of thumb is 1-2 feet.

There are two possible answers, you can select your fuse based on the maximum rated amperage capacity of your power wire OR based on the maximum current draw your audio system is capable of. The latter though must also be less than the cable’s maximum amperage capacity.

Yes, actually you would need two inline fuse holders in a dual battery installation if Battery A is under the hood and Battery B is in the rear of the vehicle. One inline fuse holder would not be able to protect your vehicle because with a second battery, power is already on both sides of the fuse at all times. An inline fuse holder should be placed near each battery protecting the cable that is passing through the interior of the vehicle in case of an accident.