If you're installing a subwoofer(amplifier) then its better to take the feed direct, via a fuse from the battery, few cables in the car can supply enough power for a decent amp. Whilst it may "only" draw 10/15 amps normally peak draw can be twice that, bass sucks power from the amplifier.
Either get a sub wiring kit( best and not too expensive ) or use 30A cable, fuse should be no more than 30cm from the battery.