The side and tail lights, however, which you may need to leave on when the car is parked, are always wired independently of the ignition switch. When fitting extra accessories, such as a rear window heater which consumes a heavy current, always wire it through the ignition switch.
The other, probably rated at about 20-30 amps, protects components not wired through the ignition — horns, interior lights and the cigarette lighter. Where a single fuse protects a number of circuits and keeps blowing, each circuit must be checked individually to discover which one is faulty.
Modern cars often need room for many wires in confined spaces. Some manufacturers now use printed circuits instead of bundles of wires, particularly at the rear of the instrument panel.
If the battery voltage drops, less current flows, and eventually there is not enough to make the components work. The extent to which a wire resists the flow of current is called resistance, and is measured in ohms.