Ok, here is the testing results. Pretty much what I expected.
Before I start I will say that it looks like I may be right and wrong at the same time. I think the problem is SOMEHOW a ground problem, but only on the right turn signal/stop connection. And only on the connector, and not on the truck's lighting.
Here is the connector. I have numbered each contact. Even on the built in 4-flat connector.
You must be registered for see images attach
1 is running lights.
2 is left turn signal and stop.
3 is ground.
4 is electric brakes.
5 is right turn and stop.
6 is constant power.
7 is right turn and stop.
8 is left turn and stop.
9 is running lights.
10 is ground.
The results...
Continuity tests with the truck turned off, all lights off, etc...
C between known ground, (Bumper.), and #s 2, 3, 8, and 10
C between any combination of the above numbers.
NO continuity between 5 and anything else.
Test with analog test light.
With the truck turned off, and no running lights on...
With brakes on... #s 2 and 8 light up. Not # 5
Brakes off, hazard on. 2 and 8
Left turn, 2 and 8
Right turn, nothing
Running lights, 1 and 9
Key on, nothing else, # 6. (#6 does not light up unless key is on. So that 'constant power" is only when the key is on.)
Every light on the truck itself works exactly as it should. Absolutely no problems there.
This testing cuts out the trailer. But remember that on the trailer, any time that right turn, or stop light is activated, the complete light on the right side, turns off. No dimming no brightening, no blinking. It just turns off.
Ok, so what this tells me, so far...
It would be logical, without that last bit of info, to assume that there simply isn't any signal from the connector to the trailer, for the right turn signal/stop contact.
Nothing at all seems to be happening with that contact. No ground. No continuity. No light. Just zero response for anything at all on that #5 contact.
But if you add the last bit of info, then it is obvious that SOMETHING is happening through/with that contact, because the running lights come on, on the trailer just fine, but that right light gets interrupted when the right turn, or brakes, or hazard are turned on.
So, the problem is isolated to the wiring going to the right turn/stop contact in the connector.
Maybe it's a ground problem. Maybe the wire is grounded out somehow. I don't know.
A grounded out situation seems precluded by the fact that it doesn't present as a ground in the testing. If it were shorted to ground somehow, then it should
ALWAYS present as a ground, in the testing.
Instead, my gut is telling me that it is the lack of ground that is the problem. When that circuit is activated, there is no ground for it to go to. and it gets fed back through the wiring, instead. Which actually turns off the light.
But that doesn't seem to make sense either. It seems to me that the lights themselves are providing a ground.
If some sort of positive signal made it through, the light should operate.
But how is it that opening that circuit results in that light essentially getting all ground, and no power? When neither ground nor power are available from that contact?
Frankiln's explanation makes the most sense here. Even so, I have yet to be able to wrap my thinkerer around how that happens. Or what the solution might be.
I AM sure that the problem is between the contact that you can see in the pic, (#5), and the wiring on the truck. I really don't think it's the truck wiring. Or the trailer wiring. It's what runs between the truck wiring, and that contact...
Is it broken?
Is it grounded out?
Is it wired up to the wrong thing?
Those are the questions I am going to be answering for myself, when I go out there and crawl under the truck to have a look at the actual wiring...
Here's another twist in the plot.
The only extra wiring that I know of in the circuit, is a third brake light that I wired in, myself.
To do that, I ran an independent 10ga wire all the way back from the battery. And I ran a new ground wire, just for this. So neither of those are anywhere near the trailer wiring. They go to the relay I installed, that actually powers the 3rd light.
But... I had to have a signal from the brake light. So, I had to tie into the truck wiring for that signal.
The 3rd brake light only comes on with the brakes. It doesn't operate when either turn signal, or the hazard lights operate. So I don't think I somehow got tied into the right turn signal wiring.
But I guess we'll see...
Maybe I should disconnect that relay, and see what happens with this contact... First thing I'll do, right now.