Notnip writes:
"You wrote - " Circuit 20 in the mating connector is an 18 ga red wire, which splits to the two tail/stop lamps."
I asked how that was possible. Now you've changed your story.

billm responds: No, there's no story change! Notnip asked
"How can the same wire be connected to the tail lights AND the brake lights?" Which I answered.

Should I have mentioned that cars have two brake/tail lights, one on the left side and one on the right side -- hence the need for the 18ga red wire to "split to the two tail/stop lamps"?

Again ... each tail/stop lamp is shared between brake and tail light circuits by virtue of the two independent filaments in an 1157 bulb.

I wish I could paste in the schematic, but it's a multi-page fold-out and hard to read as it is.