I have the Mach Audio System, so some of this may not all help you.
I ran a large gauge wire, from my battery, thru the firewall on the passenger side, to the left of your ECU. Run the wire down under along all the trim panels to the drunk. I mounted my amp on the drivers side on the plate for the electric antenna (that Ford never optioned on the Gen4 :angry: )......Although they did leave the electric antenna connector (mach audio) which is good. There is a switched 12 volt connection for the radio that can be used as a remote turn on for the amp. Although a can't remember the color codes now

. Hook up a volt meter to each connector one at a time and have someone turn the radio on and off. If your using an aftermarket head unit, run the remote line from the radio with the amp wire. Also, connect a real good metal to metal ground somewhere in the area of the amp as well.
About tapping for a signal from your speakers, use the + from one back speaker and the - from the other. This way balance will be maintained if you every use the "balance" feature in your radio "left" to "right"(although i never do and dont really know who does).
My amp had a speaker level in connector so I ran the wires from the speakers right into the amp. If the amp doen't have speaker level in, then you will need to by a speaker level to line level converter.