[MuseChaser]

Your battery supplies 12v DC. Virtually all non-ignition voltages in cars are based on 12V DC, and the wiring is sized to carry whatever current (amperage) each circuit requires safely. Fuses are selected to carry slightly less current than the wiring, so that they blow before the wire in the case of an overdraw and protect the wiring. Fuses, switches, relays, etc., are always on the positive side of the circuit. Nothing should interrupt flow to ground on the negative side. As the above poster said, fuses don't have polarity, just as single wire does not have polarity. A fuse is just a wire designed to melt if current above a specific amount passes through it.

AC current is not used in vehicular wiring, typically. AC current from a DC battery source would require the use of an inverter.