To keep it simple...... Signal wires eg rca's or high level inputs dont like having power wires running next to them, run the signal cables down one side of the car and the power cable down the other side. as a couple of peeps already said, run the same size earth as your power cable. I always join additional earths directly from the negative terminal on the battery one to the chassis and one to the motor, this stops your lights from dimming when you are pumping some serious bass from your killa sub's, and also reduces noise, i also suggest buying aftermarket battery terminals for both your positive and negative poles to cope with these extra earths and for a cleaner connection. Earths are so important and also if you install the system properly you should never have to use ground loop isolators to cut out noise.....