The '73 would have had the 400 trans, and they do NOT have a cable operated "kick-down/passing gear" set-up. It is operated by an electric solenoid inside the trans, and a switch just above the accelerator pedal. The same switch operates the cold air induction hood "flapper".
So...where was the thing unhooked? Inside the trans, or outside?

The wire for the kickdown will connect to a connector on the driver side of the trans, pointing out, just behind the shift lever.
The switch is located just above the gas pedal on a bracket. They are usually tan in color, but could also be black. It will have a small rod sticking out from the side, that the accelerator pedal arm pushes on.
The switch should have 12v power on one side with the key on. On automatic trans cars, the switch will have two sets of contacts...one set for the trans, and one for the hood flapper.
hth

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The switch should be available from most of the Corvette parts suppliers, but you may also be able to find on at your local parts house. It is a very common switch, as in, it wasn't only used on Corvettes.
The hood "flapper" is the cold air induction door at the rear edge of the hood, by the wiper area. If you look thru the small, plastic grill there, you'll see a door that goes across the back of the hood there. There is an electric solenoid the "flips" the door open when you go full throttle, and it allows more cooler air into the carb. This is why the hood has an opening in it, and th eair cleaner has a rubber seal around it to seal to the bottom of the hood. This assumes that you have the original hood, of course, and the original air cleaner housing.

Joel, (Adams Apple) knows his stuff like no other.
Welcome to the site. Gotta love them 73's.

On my 73, the trans wire comes off the back side of the throttle pedal. It is orange.