Who makes your head unit? If it is pioneer you might have to ground the rca plugs if you already grounded the unit to a clean place. 80% of the problems with noise are grounds that is why I keep bringing them up.
And yes I say pioneer because they are known for this problem. Here is a thread that might help you out more.
"Grounding the RCA's on the head unit
Many have had problems with Pioneer head units because of a poor internal RCA ground. To remedy this, simply wrap wire around the RCA's and screw the wire down to the head unit's chassis like so:"
http://www.caraudio.com/forum/showthread.php?t=198477