Sorry Don. I didn't give you enough details. I have scanned the full instructions: Microsoft OneDrive - Access files anywhere. Create docs with free Office Online.
My car has tail lights that look like this:
Attachment 54981
They are LED clusters with no inner indicator lights - only outers.
The wiring kit included did not include connectors - only pins. The instructions have steps for popping open each of the existing connectors in the rear of the car and moving existing pins (to repurpose as you mentioned) as well as insert the new pins.
There do not appear to be any spare wires in the existing connections to the BCM. I do have the appropriate tools to pop the pins out (I wired up a Helix audio upgrade last year).
I've been told there is a method for connecting the lights that avoids needing to run the new wire to the BCM and adds some additional wiring between the inner and outer clusters, but I'm not sure how that would work. Correct me if I'm wrong, but I'm guessing this would rely on drawing power from the outer indicators and the inners presumably already have a circuit to delay the blink slightly and give the "dynamic" effect.
I suspect I am best following the instructions (scanned in) instead of overthinking it.