On the new tailgate harness for the trailer hitch light, pin 8 on the main tailgate harness 8-way connector is the trailer hitch light driver circuit.
Pin 8 on a pre-2022 of that 8-way tailgate harness connector is a second tailgate latch release driver circuit. This is why the light only illuminates for a second when you release the tailgate on your truck.
You would need to add the wire to the trailer hitch light circuit on the BCM, and run it to pin 8 on that chassis connector for the tailgate harness. But, I’ve already discovered that the trailer hitch light circuit is not active on a pre-2022 BCM. So, basically, you have to tap into some other circuit to get that light to work on a pre-2022 Ram, depending on how you want it to behave.