This article is from the Car Audio FAQ, by Ian D. Bjorhovde (email@example.com) with numerous contributions by others.
The amplifiers are fine, but moving both the ground for the head unit
and the signal cables does not solve the noise problem. Take the unit
completely out of the dash, and put it on either the seat or carpet,
and run new signal cables to the input of the amp. If this solves the
problem, re-install the head unit, one step at a time and skip to level