This is an old thread from 2013, but I'm sure that replacing internal cables can be a difficult problem for many riders.

I assumed my Orbea had some kind of tube inside the frame to route the brake wire. No. It's just an empty space in there. I was stuck.

The best and easiest way to replace internal cables

I would never have thought to do this. But once I saw it, it was simple. Huh. Very elegant.

See my post with more details here.

Don't pull the old wire cable out!
Get some thin plastic tubing from the bike store. It's just a little larger than the wire itself, and the mechanics use it for this purpose.
Remove the crimp from the end of the cable, and thread on the tubing onto the old cable. Push until it comes out the frame hole at the front of the bike.
Leave at least 6 inches on each end, and pull out the old cable while holding the tubing in place.

Now you have a hollow tube that's sticking out at each end of the frame, and you can easily push the new cable wire through.
Pull the tubing out from the back when the cable is in place.

The tubing looks like this: