This part available anywhere or is it a Military part? sounds like the way to go after all especially cost wise...The 7812 is a simple voltage regulator IC in a TO-220 package.
If it's a newer-style electronic one, you need to use a DC-DC converter ahead of it, to feed it 12V.
and RGR my brake pack is a newer style but thats the reason I bought the DC DC converter mostly. I just wired up an 8 bus fuse panel giving me 8 12V splits. so here is my plan based on your info.
I will split off the military wiring harness the running lamps and signal lamps and stop lamps with the 7812's and wire in a seperate civi trailer harness to the rear chasis somewhere.
the brake pack will run off the DC DC converter receiving 12V. I will take a second split off the brake lamp line and run it through a 7812 into the brake pack to supply the trigger. Then run an independent line for the trailer brakes to the new civi harness. is BOB my uncle? kind of confusing trying to wrap my head around this one..