In the thread above there is a scan of the '69 FSM wiring diagram. The return from the reverse light switch on the 4-speed in the engine compartment goes to top connector cavity "C" as normal (this would connect to the reverse lights at the rear of the car, but with the 4-speed interior reverse light, there is a jumper than connects cavity "C" to cavity "Z" on the bottom of the bulkhead connector.
Then the inside "Z" bulkhead connection connects to the interior reverse light.