The service manual is quite clear on how-to (remove instrument bezel and upper dash trim). But it missed the removal glove box (easy) and (at least loosening the upper part) of the lower dash trim - both sides!....
You can't just take the long vent channel (behind the glove box) out, as it is screwed into the lower dash trim from behind... See pictures. I was lucky to secure a NOS cable. Now it works again.
While at it, I replaced the monaural dash speaker with a Retrosound stereo speaker....
Replacing a broken temperature control cable / 79 LeBaron
