To thread new housing through the top tube:
I insert the cable into the housing and leave an inch of cable sticking out of the end and bend it at an angle. Insert it through the front hole, push the housing through the top tube and when the tip of the cable approaches the rear hole I twist the cable on the other end. That helps the bent tip find the hole and then I grab the tip when it pops out and pull the entire housing out of the rear hole.