Calling all trailer plug (trailer to TV) wiring experts!

I occasionally tow a utility trailer with my toad (not double tow, just moving stuff). I recently discovered the RH turn signal not working on the utility trailer (all lights fine on the truck). Would have bet money the problem was with the trailer and figured I'd have it fixed in a few minutes. Well, as my luck would have it this is not the case. Trouble shot it back to the trailer plug (factory tow package) and no power on the RH TURN/STOP lug. Confirmed no incoming power for RH turn at back of plug. Since truck has separate turn/stop bulbs and trailer plugs are wired to combine them I knew there had to be a combiner somewhere and I believe I found it (them). There are two modules in the vicinity of the RH rear light cluster on the truck. I have confirmed (incoming?) intermittent (blinking) 12V to one, with no output so that seems like an "ah ha" moment, bad module. Then I tested the other one, which I have labelled with an "I don't know" and a question mark. The reason I labelled it thusly is that module has steady 12V with the taillights on. Seems to be there should be no module involved with taillights and they should be passed through to the trailer plug. Finally, the ? module also has no output.

Lying under the truck I'm kinda stumped as to the routing of power from the harness to the trailer plug. I'm guessing that all the various wires (turn/brake/tail/backup/ground) are attached as "inbound" to the light cluster, then, at the cluster connector, the turn/brake/tail goes outbound to the combiner modules then onward to the trailer plug. I'm baffled by the 12V to the "unknown" module.
Appreciate any assistance

Looks to me that the "taillight" wire should be the brake light wire.  You should have brake lights and the turn signal for that side going to each module.

The way the combiners work is if the brake light is on, it passes through the continuous light to the output.  Each time the turn signal comes on it interrupts the brake light so the trailer lamp flashes.

When the brake light is off, each time the turn signal comes on it passes the voltage turn on the trailer lamp for the duration of the flash.

The tail light voltage bypasses the modules entirely and goes to the separate filament in the trailer lamps.

Brake Light ON:
Turn signal OFF: = Trailer Light On

Brake Light ON:
Turn Signal ON: = Trailer Light Off

Brake Light OFF:
Turn Signal ON: = Trailer Light On

Brake Light OFF:
Turn Signal OFF: = Trailer Light Off

Also ... is that a break in the white wire going to the first module?

I had a Nissan with lights like that.  The module / combiner was very simple to wire and not very expensive.  It may be less confusing to simply rewire with a new combiner.  I did this back during the Regan administration, but I believe the tail lights did NOT go through the combiner, just both turn signals and one brake light in with L and R lights out.

Lou Schneider said:

Looks to me that the "taillight" wire should be the brake light wire.  You should have brake lights and the turn signal for that side going to each module.

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That's what I was thinking but showing power on the "taillight" wire with brakes not applied confused me.

The way the combiners work is if the brake light is on, it passes through the continuous light to the output.  Each time the turn signal comes on it interrupts the brake light so the trailer lamp flashes.
When the brake light is off, each time the turn signal comes on it passes the voltage turn on the trailer lamp for the duration of the flash.

Click to expand...

Thanks for the background. I've had a bunch of these over the years but never quite knew how the magic worked.

Also ... is that a break in the white wire going to the first module?

Click to expand...

Looked at the picture again and thought "oh man, there's the problem". Crawled underneath, nope, wire's fine, bad camera angle.

I'll post some more pictures tomorrow but I'm now not thinking those are combiners, I think they're taps for the RH turn and brake. I think the combiner must be over on the other side so I have to drop the spare again and have another look.

After having another look I'm now thinking the "modules" I've been look at are taps, not combiners. Looking at the attached picture I noted that:

• Wire A and wire 1 are both the same color. I believe wire 1 is power to the RH turn signal and wire A is pass-through to the bulb
• Wire B and wire 2 are both the same color. I believe wire 2 is power to the brake light and wire B is pass-through to the bulb
• Wire A1, coming from RH turn signal tap, crosses over to the left side of the truck
• Wire B2, coming from brake light tap, crosses over to left side of the truck

Based on the wire colors I'm thinking the OEM harness is modified with these taps to grab turn signal and brake light, and take them to a combiner somewhere then onward to the trailer plug. What makes me pretty sure these modules aren't combiners is that I realized there's no combining, the brake and turn signal are still separate (wires A1 & B2).

The tester I was using was an "ice pick" style with LED (occasionally I managed to stab the wire instead of my thumb  : :-[ ). When I checked wire B I was getting a steady light with no brakes, RH turn signal, and running lights on. (I was testing by myself so didn't have anyone to push the brake pedal.) I was mystified as to why I would be getting steady voltage on wire B. I'm now wondering whether the LED is so sensitive I was simply getting some bleedthrough, enough to light the LED but not enough to light a bulb. Could be a bad theory but today I will have help and will be testing with a meter.

So the investigation continues.