This is what was emailed to me from Memphis techs and it worked for me:

 

What we were suggesting was:

Tap off from the 12v power lead (constant power) and the ground <<<<

and run those wires back to the head unit. Since the remote turn on lead is going in the opposite direction (coming

from the head unit to the amp), take it and twist it around the 12v and ground coming from the amp back to the head unit.

 

The purpose of this technique is two fold;

it allows your head unit and amp to theoretically MATCH the 12v constant and the ground.

And since you're twisting those wires back to the head unit around the remote turn on, it helps cancel noise as well.

 

So, just to clarify- your head unit will no longer be connected to 12v constand and ground in the radio harness:

it will get it's 12v constant and ground directly from the amp. And since the remote turn on is running the same path

as those wires will be running, it's a good idea to twist them (or braid them) around one another.