So, for some silly reason, my first project is to put a new plug onto the trailer umbilical, updating the connection to the current standard and get the lights and brakes working. I've already got the new plug and it'll be a quick project. I've read that you can jumper from the 12v blade to the running light blade with either a short wire or a fuse and power up the running lights while parked and disconnected.
Based on my research, I've made up a chart of how the wiring should wind up:
I haven't done the work yet, so this is theoretical. If an anyone has already done this I'd be glad for see if I've got my plan right.