This is an open question for me as well. As we have discussed, I am going to be adding supplemental reverse lighting. The most common USCAR plug from CURT uses only 5 of the 7 wires, with one being the ground. You've said the blue is for the reverse, which I'd use for the supplemental lighting. So that leaves only 3 more on the 5-pin flat harness. I'm assuming 2 of the remaining 3 are left/brake and right/brake. No idea what the final of the 5 is, and I asked previously which 2 are left off the 5-pin flat. Another poster picked up from a YT video that the "red" is always hot, but I don't think Ram uses a red wire there. Certainly there's no red on the CURT harness. USCAR diagrams have not consistently reported the functions of each of the 7 wires. I find this incredible and annoying.
If it's not on the 5-pin, I don't really want to tap into a different wire. Maybe it's no problem if I stay on the CURT harness, even if it's not the 5-pin. But anyway, I would like to pull from the trailer wiring to do my bed LEDs.
I received the CURT harnesses, the same ones you use including the USCAR with 5-pin flat, as well as the 5-pin extension. I have the LUYED LEDs that multiple others have used. And I have two 6-ft LED strips for the bed. I should have time this coming weekend to wire up these projects, though it looks like we will have rain here.