Typically, induced noise will be found on power cables that are connected to A) an old battery that no longer filters as well as it once did, B) power cables that are under-sized and conducting more current than they're suited for, C) cables that are conducting extremely high current (the magnetic field created by the current in a conductor is larger when current is high) or D), it may be from something as simple as a corroded connection that should be providing a path to ground for some device that would normally shunt noise, but can't.
In any case, the vast majority of stereo system noise problems come from incorrect wiring and bad grounds. Also, for those who have read or heard about 'star grounding', this is an electrical characteristic, not necessarily a physical topology. In cars, the floor pan is often the largest single piece of sheet metal and would seem to be the best place to ground amplifiers and other items in a car but that piece of sheet metal is also providing the ground path for lights, relays, motors and sometimes, computers or sensors. For this reason, it must be seen as an electrical goal- to connect everything so that all of the equipments' grounds are at the same electrical potential, 'potential' meaning voltage (voltage is potential energy). If this goal is achieved, no measurable difference in potential exists, therefore, no voltage can be measured between two or more devices.
An easy test for ground loops- get a test light and disconnect the audio/control cables, then connect the test light to the ground terminals for the head unit and amp(s). If the light comes on, current is flowing between them and this needs to be corrected. Once the audio/control cables are re-connected, this current will be carried by the audio/control cables and noise results.
Best practice- all audio/video power and ground connections come from one source. They're not connected to any other circuitry and the signal carrying cabling isn't run close to, or parallel to any wires that conduct high current AC or DC. Also, amplifiers are located together, not far apart and the power/ground wires of each amp should, ideally, be the same length (easier to center the distribution blocks between the amps than use longer wires just to accommodate an arbitrary placement)