The diagram on the left shows the power supply cables; they are usually bought as installation kit.
All power consuming devices need at least 2 power connectors. Therefore, every battery has two contacts. To save one cable per device in a car, the minus cable is usually connected to car body. The car's body is a conductor too.
At the installation point the device has to be connected to the car body (No. 4 in the diagram). All contacts have to be good to avoid power loss at high current and the resulting heat.
The wiring cable 1 between battery and fuse should be as short as possible. Should the insulation be damaged the conductor gets into contact with the car body almost unlimited current may occur; this current might destroy the battery or even set the car on fire. In case of a short circuit the fuse (3) will just blow.
At extremely high currents a Capacitor between 500.000 an 1.000.000 uF may be added to support the battery at current pulses. Should the amplifier briefly "draw" high current the battery is then supported.
There is a rule in which order to disconnect / connect a battery: - Always disconnect cable 2 (minus) first, then cable 1 (plus). - Connecting always in reversed order, cable 1 (plus) first, then cable 1 (minus).
This procedure prevents the battery from being destroyed when just cable 2 is connected and some ground wire touches the positive pole at the same time.
The current usually flows from the minus (-) pole of the battery (No. 2 in the picture above) to the car body. This means that current is available everywhere it is needed (at least on all cars that are known to us).
That's how the current gets to No. 4, where connection is made to the car body and current is supplied to the devices (blue line).
The positive (+) pole of the battery should be connected to the devices using a red cable. All contacts must be solid and a possible fuse should be as close to the battery as possible to ensure that current flow is limited should a short circuit or a device fault occur.
Both, plus and minus pole are supplying current to the devices. Though, somehow the amplifier has to be switched on. Therefore, the control unit (usually the radio) has an additional output (remote switch-on, marked green in the picture). This output has to be connected to the amp. That way, every time the radio is turned-on, the amplifier is switched-on too.
When the car is not made of metal or the current flow is not safe, an additional wiring cable needs to be connected from the minus (-) pole of the battery to the individual devices.
There might also be cars where above connection procedures won't work.
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