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Okay guys I’m having a problem and I need some serious help! I got my truck about a year ago with some aftermarket LED type tail lamps. Nothing special, just LED boards for the tail/stop/turn functions instead of halogen/LED bulbs, and a normal halogen/LED for the reverse bulb. Well about two weeks ago I was informed my driver side stop/turn wasn’t working.

Passenger side = completely okay
High mount stoplight = fine
Driver side = no stop/turn, tail only

Well later that same day the lens blew off that same tail lamp and was gone forever, so I ordered up some 08+ tail lamps. They came in yesterday so since I have nothing else to do, I get started installing them. Passenger side unbolts, pops right out, unplug the LED tail lamps via a 3157 plug, install a white 3157 LED bulb, clip off the older 3156 reverse lamp bulb socket, solder/heat shrink the new 194/921 socket on, install a white 921 LED bulb, install the lamp back into the truck....everything is fine. Full function just like before with the LED tail lamps.

Now the fun part. The driver side. I unbolt the lamp, pops right out, and then I see it. Someone in the past has completely removed the bulb adapter AND the factory harness plug, and then direct wired the LED board to the harness via butt connectors. After sitting there for what felt like 10 hours, I made a quick run to the parts house and picked up a Motorcraft WPT109 pigtail and a Standard Motor Products S809 bulb adapter.

I get back home, check the three wires on the harness for power and ground, then I solder/heat shrink the new harness pigtail, install the white 3157 LED bulb, clip off the factory 3156 reverse lamp bulb socket, solder/heat shrink the new 194/921 socket on, install a white 921 LED bulb, install the lamp back into the truck...SAME EXACT PROBLEM! Tails work but stop/turn doesn’t!

Here is where I need the help and knowledge. The new factory harness plug I installed (WPT109) is wired up brown wire for tail, black wire for ground, and GREEN for stop turn. I pulled the passenger side tail lamp back off and the wires on that side are brown, black, and ORANGE. I have downloaded the wiring schematics that DJMAGUIRE posted in 2018, but I’m not sure all my harnesses are the same. I traced my harness back towards the cab and right behind the rear bumper is three plugs. One is a big square, 12 pin plug that goes to nothing other than the trailer lights. The second is a big, round 8 pin plug that the tail lamp harness plugs in to. The third is a small, round 4 pin plug that comes off the tail lamp harness between the tail lamps and the big, round 8 pin plug, then goes to nothing but the license plate lamps. If I keep the round 8 pin plug together but unplug the round 4 pin plug, I only lose license plate lamps.

Now if I go to the chassis side of the two big plugs (square 12 pin & round 8 pin) there is a big bundle of wiring that runs all the way up to just in front of the fuel tank on the inside of the driver side frame rail. Here those two plugs hit a junction block of sorts. The top one (square 16 pin) and the bottom one (square 12 pin) are the connectors coming from the rear. The middle one is a dead end. I’m assuming this is where the 4WD wiring would be and since my truck is 2WD, it’s just a cap of some sort.

THIS PART IS CONFUSING BUT I’LL DO MY BEST. The junction block consists of three connectors. The wiring comes FROM THE REAR of the truck (left side in picture) and connect on the front side (right side in the picture). Then leave the REAR of the connector (left side of picture) and go to their respective places. It’s almost like a big loop with connectors in the middle. I haven’t touched anything, I just know that’s how it was when I looked at it. Now of those connectors, the top (square 16 pin) goes to the engine bay with a splice in the middle for the transmission harness. The middle connector joins in with the bottom (square 12 pin) connector, and they go through a grommet in the bottom of the rocker panel. I’m assuming this wiring bundle goes up through the floorboard, behind the kick panel, and into the fuse block under the steering wheel.

Here is the kicker. If I unplug the round 8 pin plug behind the rear bumper, I lose all rear lights. If I keep that plugged in, but I unplug the bottom (square 12 pin) connector at the “junction block”, I lose all rear lights.

I’ve attached some pictures of everything and would love everyone’s input no matter how much or little! What would you do and why?!
 

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Ok let's trace the circuit starting at the turn signal switch in the dash. My wiring manual is for a 2000 truck but all 7.3s SHOULD be the same, at least for this (I'd hope).

  • At the multifunction switch on the steering column, power is sent to the left rear through a LG/OG (light green with orange tracer) wire to connector C1049 which is a 40 pin black connector to the left of the steering column. Does the LR turn light work with the hazard switch? If yes, bad multifunction switch. They output on the same wire but use different internal circuits.
    • Here it branches to the battery junction box and eventually back to the trailer adapter as a yellow wire. Do you have signal power on the yellow wire at the trailer plug? Also check the fuse for the camper adapter (location varies by model year but is found in your manual). A bad fuse here isn't the cause of your issue, but could help diagnose.
    • The other branch continues as LG/OG to connector C1045 which is a black 16 pin connector under the brake booster, 2nd closest to firewall in a group of 4 connectors.
  • From here it goes to connector C401 which is an 8 pin connector in the left rear of the vehicle by the bumper.
  • Only other connection is C431 which is the plug to the bulb itself. The same LG/OG wire should still be carrying the signal at this point.
So with that said, it looks like a direct line from under the brake reservoir to the rear bumper according to my manual (Ford factory). So If you have output power from the switch when in the left turn position on that LG/OG wire, there must be a fault in the line. You could try isolating it at those connectors and check for continuity along each wire run, then across the connectors themselves. If that fails and you definitely have power at the switch, I would cut that wire in the engine bay and run a new wire directly to that left turn signal pigtail. Not the prettiest solution, but this is a small problem to spend a lot of time and money on. And just for fun, plug a normal bulb into that socket to make sure it's not something in the light themselves.
 
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