That sounds like all electrical/wiring issues, unless the amp for the speakers inside the head unit is shot. I would try taking two of the wires from the harness (say, the white and white with the black stripe), and hooking them up to a speaker while it's still hanging out of the dash. If it works then, there's gotta be wiring issues further out in the car. If it doesn't work directly from the back, no matter what, I'd say you have a bad deck and should send it back or at least call Pioneer. They're a big company and should have at lease some technical support for ya.
Good luck!