The rear plate is installed before they press on the rear and center pusher, and it is actually free to slide on the cable between the two pushers. The front plate is also free to slide on the cable, so it could slide off the front end of the cable.
The plates are the same shape as the inner parts of the tubes, just slightly smaller to fit into the tubes.
To install, you place the rear pusher and the plate into the top tube, slide toward the front. Then install the two springs and push them in as far as the can go; then mount the spring conduits back onto the corners. (Make sure to place the cable between the two springs; hard to explain, but if ever doing it, it is easy to understand. The springs are actually welded together at the end where they go into the tube.)
You then take the cable through the pulley, and plenty of room for the pusher and plate to fit through the pulley bracket.
If not done previously, now you can cut the original cable, by pulling it back until the pusher and the plate come out with the cable, still leaving a lot of cable hanging out the front. When I did this, the plate would not come out, due to the channel having been well greased for many year, the cable just slid through the plate. So I pulled it all forward and attached a small tie strap to the cable, and then pulled on the back, and the tie strap caught the plate and pulled it out the back.
I cleaned the old cable with degreaser, and taped up the old cable with electrical tape, cut the cable in the taped area with a Dremel and a reinforced cutting blade. I then placed a heat shrink tube on both the end of the old cable and the new cable, shrank it with the heat gun, and then pulled the old cable from the front and the new cable slid right through the channel and out the front.
That's as far as I got since it was then I noticed the the center pusher had not entered the cable and yet I could not pull the cable any further to the front since the rear pusher plate was up against the springs.
There are some brackets on the ends of the conduit that are held in place with a vertical bolt for each bracket (2), and a tie plate that ties the two conduits together with two horizontal bolts.
As maccve stated, it is not a hard job; providing the cable is correct. However, it is one heck of a messy job due to all the grease that has been pumped into the tracks over the years. I threw away the shirt I used it was so covered with grease.
Just got a notification, the shipped the new cable via UPS 2nd day air; which I will say is appreciated greatly, and I'll let them know.