What would happen if you pull a double pull-line through the rigging tube (to be prepared for step 2), then use one line to pull the new cable into place from the OPPOSITE direction? I haven't seen your setup, but I wonder if the "other" end of the cable could make it through that bend more easily when fed from the opposite direction. Did I make sense?
Step 2 would be to use the second remaining pull-line to pull the other cable into place.
The reason I'm pulling from the direction I am is because the peg on the cable that connects to the motor side is about 1.5x the size as the one on the remote control. But you might be right, It might just not be possible to pull it through the direction I'm pulling it. I'll try pulling it the other way with a fish tape.