Yabut you are not getting the "full story" ALSO AT THAT SAME WEBSITE is a 73 full Mopar service manual, you can download free. Those wiring diagrams, while useful, and sometimes easier to follow, leave out details and options, and do not properly diagram connectors.
73 Dodge Body and Chassis service manuals.
[URL]https://mymopar.com/service-manuals/[/URL]
Might interest you to know that that manual, and a few others on that site, got there because of some of the guys right here.
I don't know why the harness you bought would NOT have provisons for the wire to the start relay, or maybe they sell the transmission harness separate? Here is how that works
On the cars built 70 and later, either stick or auto use the same starter relay. The start signal comes off the key switch (usually yellow), goes through the bulkhead connector to one of the small "flag" push on connectors on the start relay. The ground for the relay coil is the remaining flag terminal. On an automatic, that wire runs from the start relay, down the firewall in the backup harness, over the top of the transmission, and to the 3 terminal connector on the trans. The outer two are backup, the center terminal is ground in park/ neutral and grounds the start relay
On a stick car, that wire comes off the start relay, through a single gromment through the firewall, and connects so the clutch switch, where it grounds with the pedle depressed.
To test the thing otherwise, just use an alligator clip lead and ground that terminal, see if it cranks with the key. E CAREFUL it is in neutral/ park, etc