This thread really helped me out today. One of my shades needed restringing after the cord snapped. I ended up using bead wire -- my first choice would have been 50# fishing line but for various reasons it was easier to get bead wire. It works great now, smoother than the one that hasn't broken yet.
I don't have the plastic cord locks -- on mine the cords wrap around another channel below the window, and join with a second spring. They're otherwise the same, though, and the diagram was a big help. Until I saw that I just couldn't visualize how they worked.