New Radio Antenna Cable
Despite getting a clean bill of health on the radio, the reception was still dropping out. I decided to replace the antenna cable to see if this helped. The antenna cable fitted to the car was not original, it was much too long (perhaps from a later model Corvette) which meant it was coiled up under the dash.
I purchased a new cable from Zip Corvette, and fitted it at the same time as the blower motor cable (given that both cables run along much the same path).
I also installed a new grommet where the cable goes through in to the wheel well. The original grommet was missing entirely, so the cabin was not sealed in this location.