This article is from the Car Audio FAQ, by Ian D. Bjorhovde (email@example.com) with numerous contributions by others.
We have proven that the amplifiers are good, the head unit is good, and
the car's electrical system is good. Now we need to reconnect each
signal processor. Repeat this level for each signal processor used in
your system; if you have added all of your signal processors, and
there is no longer any noise, CONGRATULATIONS! You've removed the
noise from your system! Connect the signal processor. If there
isn't any noise, then go on to the next signal processor. Otherwise,
try re-routing the signal cables. If this cures the problem, the route
them permanently over the quiet path, and install the next processor.
If not, then isolate the processor from the car's chassis except for a
single grounding point. If this works, then permanently isolate the
processor, and move on to the next processor. If isolation does not
help, then advance to level 6.