I was just about to start a thread on this as I am currently working on mine.
The rear unit did not drain and I followed a suggestion to hook up a vacuum to the drain tube exiting in the engine compartment left side. Did the same with the front. I could hear the air hissing inside but no luck they did not drain through the pipe but still on the roof.
I then opened the rear unit and the piece of tubing going from the T-Connector the the drain cup had dried up suff in it. I squeezed it by hand and now it drains just fine.
However in the front I found to my surprise that the drain cups and tubing to them was completely missing.
The pex tubing runs in a separate channel inside the roof above the roof truss. So in the foam section between the metal truss and the outside roof sheet.
In the rear there is a 90 elbow right on top transitioning from horizontal to vertical down. If this is the case in the front as well then I think it would be very problematic to pull a new tubing as the elbow will not make it down the window channel.
Idea ... maybe you can insert a smaller tubing into the existing one and try to run that up ... needs to be flexible though.
Why do you think you need to replace it Pex is pretty much indestructible?
Here a pic from the front AC open
2006 Alpine 34 FDDS
2013 Honda CRV
Blue Ox, Patriot