I take no credit for this other than my uncanny ablility to cut and paste.
2.5 I heard that I should run my power wire directly to my car's
battery. Why should I bother, and how do I do it? [JSC]
===================================================================
For some components, like head units and equalizers, it's acceptable to
use the stock wiring for power. However, amplifiers generally require
large amounts of power, and accordingly will draw large amounts of
current. The factory wiring in most cars is not designed to handle
large amounts of current, and most wires have 10-20A fuses on them.
Thus, you will almost always want to run the power line for your
amplifier directly to the positive terminal of the battery. This could
require drilling a hole through the car's firewall, or at least
spending time hunting for an existing hole (the steering column is a
good place to start looking). Always remember to place a fuse on your
wire as near to the battery as possible! For various reasons, such as
an accident or simple wear and tear, your wire's insulation may
eventually crack, which could allow the conducting wire to make contact
with the chassis of the car and short the battery through this wire,
which could lead to a serious fire. The closer you place a fuse to the
battery, the more protected you are. Also, when running wire through
areas with sharp metal corners, it is a good idea to use rubber
grommets to provide extra protection against tearing through your
wire's insulation.
2.6 Should I do the same thing with my ground wire, then? [JSC, IDB]
=======================================================================
No. In almost every case, the best thing to do is to ground your
amplifier to a point that is attached to the chassis of the car and is
as close to the amplifier as possible. The ground wire should not need
to be more than about eighteen inches long, and should be at least as
large as the power wire. The point to which you make your ground
connection should be an unpainted piece of bare metal.
Some cars (Audi, Porsche) have galvanized bodies, and in these cars,
you must find one of the manufacturers' grounding points or else some
noise can result.