New Rear Blower Vent Cable
I finally purchased a replacement Rear Blower Vent Cable from Zip Corvette. Replacing the cable requires removal of the blower cover panel, b-pillar trim panel and kick panel. The cable runs along the sill panel under a metal cover (hidden beneath the carpet) that also houses the radio antenna cable and the wiring loom for the rear of the car.
The replacement cable is quite a complex part, it includes the under-dash knob and bracket and the brackets to mount the cable end to the blower switch.
Once fitted, the new cable worked perfectly. It's now possible to control the blower motor speed using the under-dash knob.