NEOHIO’s harness is awesome. Using it to power my radios behind the rear seats (have the longer ones).
I’ve basically got your setup, I have my Mini on the Swarfworks 3rd Brake Light, so my power cable comes into the cab that way for the unit. I run it down through the passenger sill channel to the passenger footwell. I have a 12V -> 36V upconverter tucked in there (Mini will pull up to 36V during boot up//heating cycles, so having it there prevents potential brownouts during those odd times), and with power coming from the passthru, and then it just grounded there.
There’s actually 2 different gauge wires there, I use the larger of the 4 for my Starlink, Chase Lights, and eventual compressor activation. It’s all below 5A, and the section is short. So far so good.
The interior passthru wires are in that passenger footwell, taped with another bundle. There’s a white label wrapping it, so you’ll know it from that. They’re not conventionally named as passthru 1-4, but instead just a random string that’s matched on the under hood end.
(Can’t find my photo. :-()
The Underhood end is taped on the big bundle of wires coming out by the upfitter box, down from the grey caps and covers to the computer there.
Link one end to your upfitter 6, and make sure you flip the fuse over if you want constant power.