Bike mechanic magic!

On many places a cable can got through, there is a place where the end of a cable can get stuck when you try to push is through. Think about the end you put on the cable housing for instance. Once the cable end is through, no problem. I've found that just giggling the cable against what ever is stopping it will usually help it find it's way through. You'll get the feel of it after a doing a few hundred.