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Yea I would check for power on it and if it does when this happens like when backing up, etc.
Does the truck have a electric brake controller? I don't think so as they came out a few years later.
Any way if you have the left brake/turn, right brake/turn, running lights.
The only others I can think of would be ground (most of the time it is white or black on the trailer), back up light.
If it had a trailer pk then could have trailer brake & a power line to charge a batt. on the trailer. Most of the time the brake & power are heavier gauge wire.
That is all I can think of at this time.
Dave ----
I looked at that Gary but none of the colors match. Ford's trailer wiring was a mess back then. They seemed to change it every year. Some years had different colors, some years had 1 relay some several relays, then they would not have any relays. Unless you have the exact year factory diagrams, its hard to keep track of the trailer wiring.
Dave - The red looks to have a stripe, which could be green. So, that would be a match. And there looks to be a white and a yellow. But the 4th wire doesn't appear to be black. But, 3 out of 4 seem to match. What am I missing?
Having said that, yes Ford's trailer wiring was a mess in that era. And the EVTM doesn't really answer all the questions. But, if the OP would check those wires he might discover that three of the four are correct.
On the other hand, if the rest of the wiring has been done the way the blue/white ground in the right of the picture was done then there's no hope.
Well, I checked the colors and I was wrong due to fading.
Correct (at least what I can tell now) is:
Light green/black (thicker gauge than the rest)
Red/black
Light green/white
White/light blue
I have to check if the red/bk is backup which I assume from your answers.
The blue/white "ground" wire you see in the pic was just for quick testing with a bulb to ground to see what wire does what. It was better visible from the cab than the meter