Electrically speaking, your car is a pretty active place. Every piece of electronic equipment in your car, such as power windows, your windshield wipers, and the alternator, generates its own small electrical field. These fields can be conducted through the metal that makes up your car's body and chassis. They're not really a problem until they sneak into the audio components of your stereo system and become noise.
You know what noise in a stereo system sounds like: it's that high-pitched whine that often gets louder as the car goes faster. It's generally due to electrical radiation being conducted by the audio cables of the vehicle's stereo. The threat of noise increases when you add an external amplifier, as the presence of more power- and signal-carrying cables translates into more potential intrusion points for radiated noise. It can be a pretty pesky problem, easily introduced, but difficult to pinpoint and eliminate.
Fortunately, there are steps you can take when installing a new amplifier to prevent the introduction of noise.
Grounding — An important factor in preventing the intrusion of noise is to make sure your amp is solidly grounded. Your amp's ground wire should be fastened securely to a section of bare metal, and connected tightly to the amp itself.
Isolate your amp — If you have a strong ground, yet noise still seems to be a problem, try mounting your amp on a board, then installing the board in your vehicle using rubber grommets or feet. This can help isolate the amp from any electrical noise currents that might be conducted through your car's metal panels.
Wiring — Never underestimate the importance of well-insulated, high-quality wiring as a weapon against noise. Cheap wiring tends to fall prey to noise intrusion in such a highly conductive environment (the cables you use at home won't keep out noise, because they're designed to function in a place that's not particularly conductive). It's best to use wiring made specifically to reduce noise (twisted-pair designs, oxygen-free copper wiring, chemical-proof jacketing, for example); the better the wiring, the less noise you'll get.
Cable routing — Don't make your amp its own worst enemy! Ironically enough, your amp's power cables can radiate noise into its signal cables (patch cords and speaker wire). When running the power and signal cables through your car during installation, you should route them as far from each other as possible (at least 18 inches apart is ideal).
Diagnosing noise problems
Use this diagram to diagnose and cure the most common noise problems associated with outboard amplifiers. The general idea is to isolate the specific cause of the noise in your system. If these tips don't solve the problem, it's helpful to be able to call on a Technical Advisor. Before you get started, check the fluid in your car battery. Sometimes, noise can be eliminated just by topping off the fluid with distilled water!
Mr. X — Bigger, Better, Faster