This article is from the Car Audio FAQ, by Ian D. Bjorhovde (ianbjor@mobileaudio.com) with numerous contributions by others.
Now, noise enters the system when one particular processor is
installed, but regrounding it does not help. Move the processor very
close to the amp, and check for noise again. If there isn't any, then
re-install the processor, carefully routing the cables to ensure no
noise, and continue at level 5 with the next processor. Otherwise, use
an isolated power supply to power the processor, making sure that no
part of the processor is touching the car's chassis. If this solves
the problem, the consider permanently installing an isolated power
supply or possibly a 1:1 transformer, and go to level 5 with the next
processor. Otherwise, separate the processor and isolated power supply
from the car by many feet and re- test. If there is still noise, then
there is a serious problem with the processor's design. Get a
different processor, and continue at level 5 with it. If separating
the power supply and processor from the car does solve the noise
problem, then either the processor is damaged, or your tests were
inaccurate. Repeat level 5.
 
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