I have an old steel frame that has an internal cable routing but does not have have housing inside the frame. I take out the seat post and there is a large hole for the top tube that you can guide the cable out the housing hole with your finger, it works very easily.

you can see the hole in this photo

So take your seat post out and see if you have a similar hole. push the cable in from the front of the bike and put your index finger in this hole and when you feel the cable, push the cable up through the cable guide hole.