The reverse lamp switch leads at the transmission, whether 4-speed manual, 3-speed manual, or automatic (outer two pins of the neutral safety switch on the automatic), in the 1973 wiring, routes to a grey three-way connector. The two side-by-side positions in the connector are the switch connection spots. The grey three-way connector mates with another grey three-way connector, from which the switch wires run to the "A" and "B" positions on the bulkhead connector.
The following image shows the mated grey three-way connectors:
The following image shows the mated grey three-way connectors on the car:
The switch wires can be ran directly the "A" and "B" positions on the bulkhead connector, if not using the factory grey disconnect connectors.