Whether or not the cable covering matters depend on if it still works. Operate the lever while looking at the cable connection under the dash.

If it doens't move at all - check to make sure the cable isn't seized in the housing.

- Also check the flaps in the heater box - these can get corroded and become difficult if not impossible to move no matter how hard to pull on the cable. Fixing this generally requires removing and disassembling the box.

If the cable moves some but then the housing slides in the clamp that holds it to the control or heater box, then you'll need to remove the housing and cable and free things up a bit. I used bicycle brake cable housing for new housing, reusing the old cable. (btw, do NOT use derailleur housing - they are different). I suppose piano wire could be used for new control cable if required.
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(2) '73 1800 ES 'projects', g/f's '73 1800 ES