Where do you plug in the backup light switch on a 1966 corvette Muncie 4 speed?

The backup light switch on a 1966 Corvette with a Muncie 4-speed transmission is not a separate switch that plugs in. It's a mechanical switch *integrated* into the transmission itself. It's activated by the reverse gear lever.

There's no plug; the switch is wired directly into the car's wiring harness. You'll find the wires connected to the switch on the transmission case. They'll likely be part of the larger wiring harness bundle going to the transmission area.