Need help please - question about head unit/amp installation

Hi everyone,

I am installing an amp and was wondering how to tell where to connect the remote wire for it to my head unit. It says to connect it to the DC 12v ground. Which color should that be? I have the Pioneer DEH-6400BT

I guess I am just a bit confused because I already have the black wire (which I would assume is the ground) hooked up.

These are the wires on the harness

Can anyone help?