Doesn't look like you tagged @DaveC with the question so I did it here.
For me, there's already a 4-pin harness in the LJ from the PO flat towing. I don't care for the pinch quick connectors he used so would redo that or replace the harness. Good news is it's routed so I can follow that.
And, I have a 4-pin on my 5th wheel from pulling a trailer with SxS on it.
Good question on the battery run down. I just installed a new AGM in the LJ. Maybe a mag mount solar charger for the road??
Or add a 7-pin (yuk) to provide a charge from the tow rig. Or a separate wire from my 5th wheel DC system to the Jeep (4 6v).
The cable is attached to th TJ's brake pedal and runs thru the firewall to the front of the TJ. I had it brought thru the front bumper. If you zoom in on the first pic, it's the little dongle if the center of the bumper.
There's a second segment that attaches between that loop and the Ready Brake unit. The only adjustments were the initial setup, getting that connecting segment to the length that resulted in the level of sensitivity I wanted. Took maybe 3 trips around the block and 15 minutes. Second pic shows the connecting cable. Not the best of but you should be able to get the idea.
Safety cables (6k lbs each) were bought from etrailer as were the magnetic tail lights. Lights plug into the truck's towing outlet and I run the wires along the hood (inside the latches to prevent rubbing) then in thru the passenger door and out the tailgate to sit on the bumper.
Key in the TJ is in the unlocked position but not on. There are 4 key positions; off, unlocked, accessories and run. This allows the front wheels to steer with the tow rig but doesn't turn on the battery.