In the noisy and imperfect world of car audio, proper grounding is essential to the SQ and lifespan of your system. In this paper, I will outline proper grounding techniques.
First of all, make sure that your ground cable(s) are the same gauge as your power cable(s). Remember that all of the current that your amps are drawing has to go somewhere, and that somewhere is ground.
Second, make sure that your ground wires are as short as possible.
Third, when attaching the ground cable to the ground, make sure that the spot you choose is bare metal, not painted, and that it is connected to the frame. Checking continuity between the ground (black) post on your battery and the ground point you have chosen is a good idea. You should also consider upgrading the ground from the battery to the chassis, specifically, running a very stout wire to the firewall.
Finally, when grounding multiple amplifiers,
try to using the same ground point for each of them. This will minimize ground-loop
problems. If you have a multiple amp setup with ground-loop interference, try moving the
grounds around. This should clear up your problem. If not, consult your friendly local
teamROCS member for help.