Yes like you said it will only go in one way. It's drove us mad today in and out and the same problem every time.Has the cable been wound on the drum in the correct direction? I'm sure it'll only wind one way, but that's all I can think of, off the top of my head.
Disconnect battery for 30 secondsHave you tried to calibrate the rear window? This needs to be done, or it can't work correctly.