[Qmulus]

Personally, I would just forget the factory wiring and run new wires to the dash, doors and back (if you have rear speakers), if you are going to just use the speaker outputs from the head unit. That way you can also leave the factory wiring alone so if you want to put it back to stock for some reason, it would be easy to reconnect.

The only kind of tricky part is the door speaker wiring, but you can get at the wires right at the A pillar where they go into the door connector. They are pretty easy to find as they are the only large twisted pairs of wires going into the door. If you cut and splice into them a few inches back, it would be easy to undo in the future.

Right door: Violet (+), Violet/Brown (-)
Left door: Green (+), Green/Brown (-)

I have an '03 with the MOST optical bus for the input to the amp and CD changer signals, so I pretty much have to rewire if I upgrade the head unit. Running new wires to the speakers is FAR easier than messing with running wires to the amp connector.

FWIW, I worked as a pro stereo installer through college. At that time I always had to have the latest and greatest. Now I just leave the OE stuff and listen to FM with the top up and the engine with the top down...