Ugh! No tips for your Trek Composite. But whenever I dealt with internal routing (my GT, Vitus, Techniums), if the cable housing is ran full length through a tube, and you desire to replace both cable and housing....
cut the ends off the cable and secure it stationary. pull the housing out leaving the old cable as a guide for the new housing. Slide the housing through the tube.
Then secure the housing and pull out the old cable and replace with a new one.
This takes minutes. Without a guide, it'll take hours fishing and poking.

Good luck!