73 340 duster reverse lockout light wiring diagram

My Mopar doesn't list a FSM for the 73 Duster, maybe a member has the diagram and can post it for you.
Dodge, not Plymouth. That is true of several of the manuals on that site

If you have not used the 73/ later factory diagrams, you are in for a good/ bad education. In that book, the other models besides A bodies are all the same old line drawings. But in 73, the A bodies, Ma went to a new system. CE40 is in the pages after the diagrams. This stands for "Connector, Engine Bay" no. 40 You can see there is a note giving you a clue as to it's location.

The other is a clip out of the diagrams. Notice CE-40 at the top. That wire goes through the firewall with an individual grommet, and connects to that CE40 connector out in the engine bay.

reverse2.jpg

reverse1.jpg

Then, in the one CE26, you can get another clue of how this is set up CE26 is also in the engine bay, the individual wire coming out from the lamp, and with the other end, B2-18BK going on to connect at CE40

CE26.jpg