I didn't see this addressed anywhere.
My car has the premium sound upgrade. I replaced the head unit today and wired in an amplifier. Has anyone seen the wiring harness behind the headunit on the upgraded models? The normal unit plug goes into a spliced harness which then connects to the factory radio.
My guess would be that the add-in harness feeds power and signal to the amp/sub under the passenger seat. Is that all it does? Or are the door speakers also wired into that new plug?
I bypassed that piece and wired my headunit to the normal plug, and then wired the speaker outputs from the amp into the speaker wires on the plug. Is this the correct method?

 

question has been answered elsewhere.