Heater cable replacement

[Jimmers74]

69 Roadster Need to replace temp cable and housing, which is the far left lever. Suggestions? It is difficult (impossible?) to reach the retaining screw and nut up under the dash. Bent end of cable is broken off, so total cable is too short. Not enough room to work in new cable and attach bent end to lever. I would appreciate your input. And while we are at it, what is the size of the screw that holds the cable at the valve? I've tried both std and metric with no luck.