Ooooh, I can help with this!
So I got really paranoid driving my deuce at the blistering pace of 52 mph in Atlanta with everyone else going 80 (or 100 on 285) so I decided to mimic the M939 series marker light setup on mine. I am still debating whether to copy the M35A3 setup in the rear, with the 3 center mounted markers and 2 outboards. I just hate to cut my nice factory bed.
Anyhoo........................
For the front, I used the military 2-into-1 adapters and I pulled power from the low beam of the headlight. I figured if I was somewhere empty enough for the high beams to be used, I didn't need the marker lights. So one of these on each headlight to feed each side.
For the rear, I mounted the "middle" (front of the bed) lights and ran the wiring along the front edge of the bed, towards the drivers side. Both wires terminate sort of near the airpack. You can see them already wrapped in their asphalt covered cloth wiring harness wrap here, along with the wiring for the various spotlights I equipped the vehicle with.
Once the left and right marker light were tied together into once circuit (soldered them together into one wire), I ran the single feed wire to the rear of the truck, inside the frame, inside the factory wiring harness clamps. Of course, this feed wire is wrapped in the cloth wiring harness wrap/sheath/whatever to protect against abrasion.
When I did the rear marker lights, I was also in the process of doing reverse lights. I built a sub-harness with 2 wires (low and high beam) for the reverse lights (high beam is wired to the Peashooter/Gringeltaube switch on the transmission) and low beam is wired to a switch on the dash) and a third wire for the rear marker light. Using the aforementioned military Y connector, I also tapped into the taillight circuit on each side and ran the power feed through the harness as well.
The power taken off the passenger taillight feeds the rear marker lights and the power taken off the drivers taillight feeds the middle marker lights. The left and right sub harnesses terminate behind the trailer connector like so:
So one taillight feeds the rears and the other taillight feeds the middles. Obviously we're talking about circuit 21.
Everything is soldered, all connections are new packards and all the wiring is wrapped in Grote's cloth covering like what the factory used on the front turn signal harnesses.
Like this:
Now, I'll bet somebody is going to say "You're going to overwork the headlight switch, running all those additional marker lights on the 21 circuit, all of which is fed from the headlight switch".
Several things:
1. I ran it 6 months on the original headlight switch with no problems. Even on a long trip (lights on for 8+ hours, Athens to Charlotte, non-stop) the switch didn't overheat. I ended up with a new 3 lever switch and swapped it just to be safe since my original was 25+ years old.
2. The M35A3 comes from the factory with 7 marker lights running on the 21 circuit all through the 3 lever switch.
3. When the military hooks up an M939 5 ton tractor to a 40'+ trailer covered in taillights and marker lights, all those lights are fed through the 3 lever switch (without any relays).
The finished project: