Funny, I need to do this exact same thing on my '84.
I got a cable, clip, and heater valve off an '87 2WD, it's all the same.
Only now I've lost the clip and I'm really ticked about that because the only way to get the clip out without risking breaking anything is from the top, and you can't get to the top of the heater controls without taking a bunch of other stuff apart. I was just going to cut the old clip with some wire cutters but now I might have to reuse it...
If you can get to the top of the heater control, getting the clip out is pretty easy with a small flat blade screwdriver.
That one clip on the heater controls is the only clip holding that cable in place on my truck. I intend to tape the new cable to the end of the old one and just use the old one to pull the new one through the dash, just like fishing wire through a wall.
Where are you located? I have an extra clip, it's the first one that I messed with to see if it would come out from the bottom so it's a kinda beat up but it might still work for you. Let me know if you want it.
