depending on the cable (if it has 'solid' parts, and at which end) you can tape the new cable to the old one, or tie it together with wire, and use it like a guide wire to pull the new one threough.
which direction depends on the situation.
With a bit of skill, most of the time it is possible (and not too difficult) to just poke the new cable through the frame (it is stiff enough to be guided. Believe me, these cables have to be replaceable, and easier than iPod repairs!! :) good luck