as for electron flow and the car chassis, the car chassis is made of steel while power wire is usually made of copper. copper has lower resistance than steel does, but because of the amount of steel in a car's chassis/body, the car body usually has a lower resistance than a wire running from the trunk. using the car's chassis also has it's advantages as opposed to using a ground wire connected to the battery.

ALL the currents in the car pass through the battery post, so all the noise does too. if someone wants a high high concentration point, look no further. this is a terrible place to ground equipment from a noise standpoint, and since the chassis is so large and as suchlow resistance, it is sensible to use it. there is nore to it than that, however, and ground loops come into play here. if the potential difference of all the ground planes in the equipment is the same, then there will not be ground loops.

as for noise in the chassis, this one is a bit more complicated. the chassis is not perfectly uniform from an electrical standpoint because there are impurities in the metal, etc. this means that electrons find a path to run from wherever something is grounded to the power supply source (the Alt and battery posts), and we can't look at a chassis and tell where these paths are. there are high current concentration points and low current concentration points in the chassis and of course this is dependent on where the currents in the chassis flow. the above also determines high noise and low noise areas as well.

it is also true that running power and low level signal wire causes no noise in and of itself. it doesnt matter if they are running together because anywhere you put the wires you will be close to the chassis and as such close to noise. once again, separating wires makes no difference.

the noise issue arises usually when there is a "ground loop". a ground loop occurs when currents are given more than one path to ground and the potential difference between the two are non zero and there is enough noise present to be induced audibly into the system.

someone mentioned that the headunit should always be grounded in the harness. this usually doesnt matter but if there is noise in the system this is the second thing i change. the first is my amp/processor dristro block ground location. again, there is no way to know where there is going to be a concentration of noise in the car chassis without testing.

car audio systems these days are more often than not designed to lift the ground of the audio inputs away from the ground plane of the power wires. this helps to diminish the level of spurious noises being induced into the wires that are running down the car. at times we have noise even with all this fancy design and this is where experimentation comes in with grounding locations, etc.

winslow is correct on the shielding issue. as for dynamat, this makes no difference because its not connected at one end of the chassis(like a shield would be) and then left floating at the other end while trying to pass signals to your audio equipment .



all the issues above have been discussed numerous times on most forums and with a few conclusions:

running power wires and low level signal wires together will not have an effect on noise. there is no statistically reliable evidence to show that it does matter, but there is a lot of evidence available to show that it doesn't.

using shielded RCAs are usually a bad thing.

there is no need to run a separate ground wire from the trunk to the battery(simple math shows this to be true).

when noise is present in a system there is usually a ground loop involved.

An older mechanic at my dad's shop once told me "since it takes a positive and negative connection to complete a circuit and the negative post of the battery is connected to the chassis making it part of the circuit no matter where you run the other wires they are always next to the power wire". Made sense to me so I just bundle 'em up and run them the easiest way possible in most cases.