Hi Ron, If I understand what you are talking about,That tube controls the rear comparment floor heat on the right side only. If you look at the left side, you can see how it funtions. (providing ITS intact . This at best this was" make belive" heat. These cables were controlled by the left lever (by the gear shift lever) . These cables have nothing to do with your main heat cables. If you are determaned to replace the cable, I think Ace Hardware use to have a 5/16 in. steel tube that might work or find someone with a old VW pan that you could rob. Frankly , I wouldn`t worry a whole lot about it, but its your coal truck and you can haul anything in it you want to. Good luck on the pan job.
   Press On !!
   David

Ron,  Just how much of the tube is left (tunnel side) ?  I have seen these cables going through the air to the "flap" lever after the tube has rusted off with no problem, but if it comes up too short and you want to replace it , You can take the operating lever loose and slide the cables far enough forward to clear the bad spot. then you could weld a new piece on. You will have to disconnect the left side to move them forward also(two into one cable) If you choose to take the lever off, be sure to remember how you did it. I have seen them put back on incorrectly. Belive me , I have seen it done and with only one 13mm nut to remove. Don`t lock the nut down because this is also the adjustment for the amount of tension that is required to move the lever. It would be a good idea to squirt a little " nut buster" into the tubes at the front and rear while you have everything loose. NO NO NO !!!! NOT W-D 40. Anything but that. W-D 40 collect water. Rust City. I hope this helps you out and I wasn`t to confusing.Keep me posted
 PressOn!!
 David

Ron, If you choose to do the "air route"  Be sure to put something on the exposed cable. That battery is just looking for something to "chow down" on
  David